In World War II this need for military underwater security was first shown by the achievements of frogmen against armed forces facilities: see for example Italian frogman actions in WWII. Since the late 1950s, the increasing demand for and availability of sophisticated scuba diving equipment has also created concerns about protecting valuable underwater archaeology sites and shellfish fishing stocks.
The 12 October 2000 USS Cole bombing was not carried out by underwater divers, but did bring renewed attention to the vulnerability they present for naval ships. Divers can swim 100 to 200 yards in three minutes time, and large sonar ranges would need to be established around ships in order for security forces to detect underwater swimmers in time to make a sufficient response.
In March 2005 the Philippine military, interrogating a captured anti-government terrorist bomber, found that two of Southeast Asia’s most dangerous terrorist organizations linked to Al Qaeda were said to be jointly training militants in scuba diving for attacks at sea.
Hereinafter, "nlsn" = "Non-Lethal Swimmer Neutralization Study"
Following World War II, the increasing popularity in recreational diving introduced a new complexity to underwater security. Divers must not only be detected, but evaluated as to their purpose or intentions for swimming in monitored areas. Steps to protect against threat or harm from divers must take into account possible reasons why they would be swimming in monitored areas. The divers may be:
- Recreational swimmers without harmful intent, or
- Poachers removing sea life or valuable objects from the sea bed illegally, or
- Threats intent on sabotage or intelligence gathering involving sensitive water targets
Swimmers can approach from the surface or underneath the waters, the two presenting their own detection and deterrence challenges. And the interception and apprehension of intruders detected in bodies of water pose unique safety risks.
There are various types of places of operation:
b) On the surface of water.
c) In small boats (e.g. RIBs) being used by unauthorized or suspect divers.
d) In larger boats being used by unauthorized or suspect divers.
e) Arresting suspect divers onshore, before or after they dive.
There are these likely theaters of operation:
a) In an enclosed security area, e.g. a harbor.
b) In open water to protect submerged valuables (usually undersea archaeological sites).
c) In open water (often on a frontier) to prevent underwater smuggling.
d) In open water to protect sea life. (This, on a small scale, may be defined to include various known unofficial actions by inshore fishermen to protect their shellfish stocks.)
e) To prevent unofficial divers from getting in the way of other water or shore users.
In most scenarios nowadays #1 or perhaps #2 is likelier, but in war or semi-war conditions or where there is a risk of terrorism #3 may be likelier than usual.
A police-type technique that is reasonably safe on land may be risky to a scuba diver.
The document nlsn leans strongly towards #1, and discusses only non-lethal weapons. But in war and semi-war situations there is more risk of #3 and the choice may be for lethal weapons.
Keeping underwater security against frogman intrusion has been complicated by the expansion of sport diving since the mid-1950s, making it bad policy for most democracies to use potentially lethal methods against any suspicious underwater sighting or sonar echo in areas not officially closed to sport divers. Any routine patrol investigation of all "unidentified frogman" reports would have had to stop because any genuine reports of intruders would be swamped in ever more reports of civilian sport divers who were not in military areas.
For a long time it would be easy for diving professionals and other experienced divers to distinguish a sport diver with an open-circuit scuba such as an aqualung from a combat frogman with a rebreather; and legitimate civilian divers are normally fairly easy to detect because they dive from land or from a surface boat, rarely or never from an underwater craft, and willingly advertise their presence for their own safety; but recent multiplication in sport rebreather use may have changed that somewhat.
However, particularly in former years when scuba diving was less common, many non-divers, including many police and other patrol and guard types, knew little about diving and did not know of this difference in diving gear, but described all divers as "frogmen"; one result was an incident in the inter-ethnic crisis in Cyprus in 1974 when a tourist was arrested for suspected spying because "frogman's kit" was found in his car: it was actually ordinary sport scuba gear.
After about 1990 the rapid growth in the number of sport diving rebreather brands has clouded this distinction, while advanced sport divers increasingly tackle longer deeper riskier dives using equipment once available only to armed forces or professionals. This means that even "less-lethal" techniques for trapping them underwater, disorienting them, or (especially) forcing them to the surface would be an ever-increasing risk to civilian divers' lives.
In former times, civilian diving was only for work, and needed standard diving dress and big easily seen surface support craft. Sport scuba diving has changed that.
Another result of sport diving is a risk of civilians independently re-developing, and then using or selling on the free market, technologies, such as technical advances in underwater communications equipment, heretofore kept as military secrets. (For a loss of military secrecy caused by independent civilian duplication, though not underwater, see Lokata Company.)
There have been incidents which have demonstrated poor underwater security, when a sport diver with a noisy bubbly open-circuit scuba and no combat training entered a naval anchorage and signed his name on the bottom of a warship. Concern at the risk of increasing the sport-diving public's ability to penetrate harbors undetected, and of unofficial groups equipping combat frogmen from the sport scuba trade, might have led to the events listed at "#Prevention" below.
The MSST (Maritime Safety and Security Team) is a United States Coast Guard harbor and inshore patrol and security team whose methods include detecting submerged divers.
A swimmer on the surface of the water is liable to detection by the same means as used on land, e.g. eyesight, surveillance cameras, thermal imaging, radar.
In WWII this was the main precaution. That is why WWII manned torpedo operations tended to occur at night around new moon when moonlight is minimal.
Open circuit scuba bubbles can make detection easy, but not easily in rough foamy sea water.
Swimming deep can hide from surface guards. If underwater visibility is good, the diver may have to go deeper than is safe with an oxygen rebreather, and with open circuit scuba more bubbles are made with each breath in proportion to (depth + 10 meters) = (depth + 33 feet).
Thermal imaging could detect a diver near or at the surface, but not so easily in warm tropical water.
Detecting electromagnetic signals in the 27 to 200 GHz range may improve detecting surface swimmers at night, but this idea is not yet tested.
Artificial intelligence and electronic neural networks and developments in ultrasound have made possible specialized diver-detector sonars.
Experience has showed that passive sonar (i.e. merely listening for underwater sounds) cannot detect everything; in particular it cannot easily detect rebreather divers and unequipped surface swimmers; and it can detect direction, but not distance unless readings from two or more listening stations can be correlated.
High-power low-frequency sonar commonly used for depth sounding and to detect large objects (including submarines) is not good at detecting small objects like divers, but the US Navy Diving Manual 24 indicates that it is hazardous to divers.
Examples of diver-detecting active sonar systems are:Sonardyne SENTINEL diver detection sonar
AN/WQX-2: the US Navy uses it.
AquaShield Diver Detection Sonar Designed to protect energy installations, ports and coastal facilities
Cerberus (sonar): a blue egg-shaped device.
CSDS-85 Omni Surveillance Sonar made by CTech
DDS 9000 multibeam diver detection sonar: by Kongsberg Gruppen
DDS-J diver-detection sonar designed to protect underwater oil pipelines: by Westminster International Ltd
Harbour Surveillance Sonar by Marport C-Tech Ltd.
UPSS = Underwater Port Security System
WESMAR Web System is a diver-detector sonar.
Underwater Security Systems
Trained dolphins and sea lions can find submerged divers. Both can see, and hear direction of sound, well underwater, and dolphins have natural sonar.
The United States Navy’s MK6 Marine Mammal System is supported by SPAWAR and uses dolphins to find and mark mines and divers in the water. This system was used in:Vietnam in 1970-71.
Persian Gulf in 1987–1988.
San Diego harbor for security during the 1996 Republican National Convention.
Animals, unlike remotely operated underwater vehicles (ROV), etc., need to be fed and kept fit and in training whether they are needed at work or not, and cannot be laid aside in a storeroom until needed.
A remotely operated underwater vehicle (ROV) could search for submerged divers; but ROVs are expensive to run, and as technology is now could not attack several targets one after another as quickly as a marine mammal.
An underwater ROV needs to be controlled. It could find and identify divers, and perhaps deter them. It should not be easily overpowered or attacked or outpaced by the suspect divers. If it is to attack the suspects, it should carry a suitable weapon.
A surface ROV can move on its own and scan below itself with sonar, but without a long-range weapon it can do little against deeply submerged suspect divers.
These links    claim that after 9/11 the FBI asked the USA's largest scuba diver certification organizations to turn over the records of all divers certified since 1998; this turning-over is now done once a year.
Some anti-frogman weapons, and weapons that may come to mind when considering defending against frogmen, are:
This is the usual method available to non-diving harbor guards, and to unofficial groups trying to restrict or prevent scuba diving in their area. For weapons, see the next section.
In some circumstances, submerged open-circuit scuba divers can be followed by their bubbles until they run out of air and have to surface, and then tackled on the water surface or as they come ashore.
According to circumstances, the patrol may need some means of transporting prisoners and/or seized diving equipment away from site.
Many casual sport diving intruders may keep away on seeing visible clearly marked patrol boats and surface barriers.
These methods may be useful when assault-boarding a boat being used by unauthorized or suspect divers, or arresting them onshore, but not often otherwise.Mace (spray) and pepper spray and teargas may make an unequipped surface swimmer drown, and are useless against a swimmer with a diving mask and breathing set whether he is in or out of the water.
Tasering a surfaced diver would either be insulated off by his rubber diving suit, or may make him panic and drown, including making him lose his scuba mouthpiece if any. Any electric-shock weapon can be shorted out by water, and also the usual design of taser's firing range underwater would be a few inches.
Bean bag rounds, rubber bullets, pepper balls, and similar would be stopped in a few inches by water.
An electric shock prod's electrodes may fail to penetrate a tough electrically insulating drysuit, or the shock delivered may be shorted out by water on a wet diving suit.
Underwater a baton would have to be used for thrusting or jabbing, not swung, due to water resistance; and the target's solar plexus will probably be protected by parts of his diving gear.
Police-type baton and riotshield tactics would be of use only onshore or in a large enough boat.
Judo throws and similar are unlikely to work underwater.
Stunning may be a fairly safe means of arrest on land, but underwater would likely make the diver lose his mouthpiece and drown (unless he has a fullface mask or some sorts of strapped-in mouthpiece), or lose control of depth with consequent barotrauma.
Underwater, a hand-held spear may have some use. Otherwise, throwing rocks, or other projectiles including sharp objects, by hand is likely to work only out of water.
Ordinary bullet-firing firearms may be useful (as a lethal weapon) against divers on the surface or men in boats or ashore, but underwater are inaccurate and very short range.
Shotguns (probably pump-action, when used as a security squad weapon) may be effective when the target is out of water, but are even less useful underwater and barrel is likely to explode.
Special underwater firearms have been designed for use underwater: see #Underwater firearms below
A depth charge is effective, and may be lethal, but may cause other damage underwater, and is not recommended in peacetime when the victim may be an intruding civilian sport diver, although it is alleged to have been common practice for some years after 1945 in British naval harbors.
Divers, however, are far less vulnerable to damage by underwater explosion than common sense would dictate. Since the tissues of the body tend to transmit the shock waves with much the same characteristics as the water around, large distant shocks have little impact on divers. For this reason, the most effective "depth charge" for use against a diver is the common hand-grenade, tossed within a few feet of the diver. The resulting gas cavitation and shock-front-differential over the width of the body is effective in stunning or killing the diver.Anti-frogman depth charge developed by Rheinmetall
Dazzlers are much less effective underwater than on land.
The Active Denial System does not work underwater, as water absorbs microwaves well (as in microwave cookers).
A magnetic field generator to make the diver's navigation compass misread is possible. Such a magnetic coil carried by a patrol boat directly over the target diver would affect compass readings to 5 meters (15 feet) depth at about 7 kilowatts; but to 10m (30 feet) (oxygen rebreather depth limit) at about 448 kilowatts, which is too much power need to be practical.
Requirements are different according to what sort of weapon is called for:Lethal.
Non-lethal, causing pain or discomfort
Audible sound giving verbal orders.
There has been much research about the effect of sound on divers. See the bibliography in nlsn (page 51 etseq, 356 entries).
High intensity sound 20–100 Hz, and high intensity impulse noise, are promising as a non-lethal weapon, but more testing is needed. As a source of high-intensity 20–100 Hz sound, the sound generated by a plasma sound source is promising.
The US Navy Diving Manual(page 24) says that high-power low-frequency sonar (commonly used for depth sounding and to detect large objects (including submarines)) is not good at detecting small objects like divers, but is hazardous to divers.
The main effects of ultrasound on the human body are heating and cavitation. See nlsn(pages 21–23) for detailed information. Also see ultrasound and sonic weaponry.
As each wave of the ultrasound passes through the diver, any bubbles in the tissue expand and contract, and the tissue heats. After a particular threshold of loudness of the ultrasound, new bubbles form during the low-pressure part and disappear during the high-pressure part: this is cavitation and can cause injury.
One well-known method is a powerful blast from a ship's ordinary high-power low-frequency sonar (commonly used for depth sounding and to detect large objects (including submarines)), which deranges the diver's inner ear and makes him dizzy and disoriented and tends to force him to surface, or may make him panic and lose his mouthpiece and drown. These large "active sonars" are used to search for submarines and are very powerful. These sonars are usually bow mounted, and if so a diver attacking at the stern would be in the sonar baffle region and unaffected, if he gets close enough first.
Most ships, both military and non-military, carry smaller "navigation" sonars such as depth finders or collision sensors, but their high frequencies and relatively low power lack effectiveness against divers.
A test of a 230 decibel 3000 to 7000 Hz transmitter killed seven whales, causing hemorrhages around their ears: see Sonar#Sonar and marine animals - adverse effects.
Around the 1970s there were reports among sport scuba divers from offshore from a Ministry of Defence area in Dorset in England of diver deaths, mass deaths of fish, and divers returning reporting "strange sonic noises": they speculated about a secret anti-frogman weapon, but it may have been merely a powerful modulated ultrasound beam intended to communicate with submarines.
Some say that these speculations are mostly fanciful and that since the human body is very close to the impedance of the water around it, the ultrasound tends to pass through the body (perhaps breaking the eardrum, but not killing the diver); but if the sound or ultrasound is powerful it may cause overheating or cavitation damage on the way.
However:This method of attack (to stun or kill) occurs in nature; it has been proved that some toothed whales can make and focus audible sound "clicks" so powerful that the whale routinely uses it to stun prey at close range.
Analysis of research literature related to effects of ultrasound concluded that reported ultrasound-caused organ damage was associated with sound pressure levels exceeding a certain intensity threshold, regardless of frequency
The UPSS/IAS diver-detector sonar system includes an underwater shockwave emitter: see Underwater Port Security System.
It is unknown what later proof or disproof there has been of speculations such as appeared in a book about Cousteau written by Philippe Diole around 1960, about underwater ultrasound guns making an ultrasound beam powerful enough to disintegrate a diver into the water except the metal parts of his kit.
To cause discomfort to the diver
A sound that irritates or causes pain. Diver aversion to low frequency sound is dependent upon sound pressure level and center frequency. Westminster International have also implemented this but they withhold the exact sound frequencies used.
The sound may be an order to surrender or surface or go onshore or to the patrol boat, perhaps with a threat to use non-lethal or lethal force if disobeyed. But such an order must be clear enough to be heard and understood.
Sensitivity to the sound
Underwater, human hearing is largely by bone conduction, through the skull and not through the eardrum and ossicles. This causes somewhat less acuity of hearing and a different graph of sensitivity against frequency, with a loss between 1000 Hz and 5000 Hz. This may affect ability to understand speech.
Research showed that, at depths up to at least 10m (30 feet), divers' wetsuit hoods lessened underwater hearing sensitivity by 10 to 35 decibels at 1000 Hz and above, and by little or nothing at 250 Hz and below. With increasing depth in a hyperbaric chamber, decreases in wetsuit hood sound attenuation appear only to occur at frequencies between 500 and 1500 Hz. In the open ocean, hood attenuation at 8,000 Hz showed a significant decrease at 60 fsw and a tendency to decrease at 2,000 and 4,000 Hz compared with the 10 fsw data at the same frequencies in the chamber trials. At frequencies from 500 - 4,000 Hz wetsuit hood sound attenuation was on average 8 dB lower in the ocean than in the chamber trials.
Underwater, humans are much less able than in air to tell where a sound came from. Research showed that what ability remains is better with bang!-type noises than with pure tones.
Research showed that loud sound at 100 to 500 Hz caused vibration, and at high powers cavitation and damage.
Sound at 20 to 100 Hz is the resonance vibration frequency range for normal-sized adult human lungs, and at high power causes discomfort from vibration in the lungs.
Loud sound in this frequency range was difficult to make, but the plasma sound source should make it easier; divers found plasma sound source noise underwater "very unpleasant".
Infrasound probably has little or no effect on divers.
A newspaper article about the Lionel Crabb disappearance speculated about underwater electric shock weapons mounted on warships to defend them from frogmen. This method, if it is used, imitates nature; see electric eel and electric ray.
Such devices occur in fiction, commonly in comics. Some sorts might be possible if designed.
Small dredging-type craft and small submarines are used for small-scale dredging and/or to recover submerged objects; but there is no known case in the real world of them being used to capture divers. The craft's capture device might be a net or a grab or an aimable suction tube or a scoop.
A net can sometimes be used to catch submerged divers.
This agrees with talk among diving circles about a fishing trawl being the handiest way for naval men to get unwelcome or unauthorized divers out of the water.
An article at the American Academy of Underwater Sciences 1991 International Symposium Proceedings says that the California Department of Fish and Game, to capture sea otters underwater for a relocation program, successfully used a net cage apparatus front-mounted on a Dacor Scooter diver propulsion vehicle steered by a diver with a silent bubbleless closed circuit oxygen rebreather. It is not known if a similar larger device has ever been used to capture divers underwater.
This type has been seen in fiction.
A text fiction story (The Deep Range by Arthur C. Clarke) mentioned a diver-catching grab used to recover a work diver suffering from nitrogen narcosis, not to arrest a suspect; presumably, it is easier to rescue an impaired diver than to apprehend an unimpaired one, but it could be adapted if combined, for instance, with a stunning device.
Grab-type devices on various scales are very commonly used in nature underwater by animals. The device is usually its jaws, but in some animals evolution converted legs into arms to handle objects; see Opabinia for a very early example of a nose turned into a grab.
There have been cases of unofficial groups dragging a grapnel behind a fishing boat through a group of submerged scuba divers.
A suction device might make an area suction effect in the open, or might be a suction tube extended at the frogman, who may be sucked against an opening and so held, or may be sucked inside.
Such devices on a small scale are sometimes used in nature to catch prey: for example by the seahorse and the pipefish, and the bladderwort plant. The mouths of many teleost fish (for example centrarchids) have a strong suction component to the way they work.
Barriers can be put in the water to try to keep swimmers and frogmen out.
There is concern that these nets could interfere with fish migration. Due to this and expense one opinion says that they are a poor choice as frogman excluders.
"Safe Barrier" make
This make is metal chain-link netting placed underwater, preventing entry into an area, or at least delaying the frogmen while they cut through it.
It was made by a Swedish company, Safe Barrier Systems (SBS), a division of NCC Stockholm. It is rigid metal netting, covered in polyethylene electrical insulation, and polyurethane abrasion protector outside that. The strands are electrified so that any frogman attack on the net will be detected by that strand going open-circuit (not to electrocute him). The grid size best suited to deter divers is 250 x 250 mm = 10 x 10 inches. Testing in the UK showed that a diver using bolt cutters could cut a hole big enough to swim through in 60–90 seconds.
It was found that the net could be evaded by climbing over it, or getting under it, or by using a wire loop to complete the circuit where he cuts each strand.
The net system can be equipped with a gate (operated by an air compressor), to allow traffic in and out of the protected area.
SBS currently supports 15 sites with "Safe Barrier" nets, including four with gates, but they are not making this net system now, due to lack of demand. The price quote for a new net was more than $7,000,000.
This make is or was made by BEI Security Systems. Its system that alarms if cut is fiber-optic.
This make was made by a U.K. company. It incorporated a system that set off an alarm when its fiber-optic mesh was cut. This make seems to have disappeared, and the tradename "Aquamesh" is now used for underwater wire mesh used in the aquaculture industry for lobster and crab traps.
These will stop surface boats from dropping divers in unwelcome areas.
One effective anti-swimmer netting to date is multilayered monofilament line wide-mesh fish netting. It is almost invisible to the diver and hard to avoid. When equipped with float sensors that detect large-scale movement, these nets have proven highly effective.
It would seem that often a simple way of countering unknown frogmen or other divers would be for a police force or navy base personnel to send their own frogmen to investigate. This is sometimes called counter-offensive frogmen. Combat divers undergo weeks of full-time underwater training, far more and harder than what most average civilian sport divers undergo; and they would be at full armed forces fitness even before the frogman training starts: see Frogman#Training. Superior underwater combat training would likely decide which two groups of frogmen would win; generally, criminal or terrorist frogmen only have access to types of training which are available to civilians, or at least inadequate facilities.
However, underwater combat between opposing teams of frogmen (although common in fiction (as in the movie Thunderball, and The Silent Enemy, and at least one incident in Sea Hunt), and often in comics) is unusual in reality.
Sometimes diving sea-police have arrested civilian divers for illegal spearfishing and diving in restricted areas and the like, and naval divers have been sent down to investigate unidentified divers in a naval harbour.
When confronted, sport divers are likelier to obey the patrol divers quietly as ordered; hostiles would be likelier to fight back.
Among the ways suggested of forcing arrested divers to surface would be attaching an inflatable float to each.
Objections to the likelihood of this tactic are:It may result in an underwater knife fight, risky to both sides.
Risk of both sides drowning because of each attacking the other's breathing sets.
This risk to the patrol divers depends on the design and resistance to damage of their equipment, e.g. kevlar-reinforced drysuit, and see Frogman#Breathing sets.
Risk of disproportionate damage to non-hostile divers by sending them to the surface too quickly, causing them to suffer a potentially lethal decompression injury.
It may be difficult for the patrol divers to find the suspects; but this depends on:
Underwater visibility, which can be from a few inches to 30m (100 feet).
The suspects using open-circuit scuba in conditions where the patrol divers can follow their exhaled bubbles.
Having a hand-held sonar of the type that has a screen, e.g. the INSS, or NuvoSonic's diver-portable diver-detector sonar set.
A trained sea mammal leading the patrol divers to their target.
If the patrol divers are riding suitable diver propulsion vehicles, they could travel faster and carry better weapons (lethal or non-lethal) and equipment for sonar search and navigation and communication, and perhaps a means (e.g. grab or net) to capture suspect divers in passing and tow them alongside back to the base or patrol boat.
It was thought expensive for a team of patrol divers to be on standby all the time kitted up to dive; but France has police divers trained to arrest unauthorized or suspect divers underwater and to force them to surface. One common offence there is or was spearfishing while using breathing apparatus.
See Frogman#Equipment for features useful in equipment of frogmen who may get into underwater fights.
The Russian PDSS system is an example of an anti-frogman defence system which includes frogmen trained in underwater fights.
See Russian commando frogmen under "1970 and after" for a report of a real underwater fight between a guard squad of Russian PDSS frogmen and intruding enemy frogmen.
The films Above Us the Waves and The Silent Enemy are reconstructions of real World War II events, and each shows an underwater fight between opposing groups of frogmen, but those fights did not happen in the real events.
Some navies have thought underwater fights to be likely enough for them to design underwater firearms for frogmen to use as a lethal weapon; there is said to have been a real incident when Russian frogmen shot two anti-frogman dolphins.
These underwater firearms fire a steel rod, not a bullet, for better range underwater. They are all more powerful than a speargun, and can fire several shots before reloading. Their barrels are not rifled; the fired projectile is kept in line underwater by hydrodynamic effects, and is somewhat inaccurate when fired out of water.For a long time the diver's standard weapon and tool has been a heavy knife.
A catalog issued in 1991 by Life Support Engineering (now Mercury Products) contained several military / commando type diving kit items and also a compressed-air powered speargun.
Underwater, a baton would have to be used for thrusting or jabbing, not swung, due to water resistance, and designed accordingly; and the target's solar plexus will probably be protected by parts of his diving gear.
A reported anti-frogman guard is (or was) dolphins trained to carry on the nose a device which injects a large amount of compressed carbon dioxide into the frogman. This would likely be lethal due to blood embolism. It is said that they were trained at Point Mugu. It was said that this device was abandoned because of fears that wild dolphins might imitate and start harassing ordinary divers. Today the mammals are primarily trained to force the diver to the surface using pushing techniques in the assumption that the majority of incursions can be addressed in this manner.
This link says that the US Navy has deployed sea lions to detect divers in the Persian Gulf. The sea lion is trained to detect the diver, connect a marker buoy to his leg by a C-shaped handcuff-like clamp, surface, and then bark loudly to raise the alarm. 20 sea lions have been trained for this at the US Naval Warfare Systems Center in San Diego. Some have been flown to Bahrain to help the Harbor Patrol Unit to guard the US Navy's 5th Fleet. Sea lions adapt easily to warm water, can dive repeatedly and swim up to 25 mph, can see in near-darkness, and can tell where sound comes from underwater. In training the sea lions have been known to chase divers onto land. See also this link.
This link reports that in 1970 to 1980 trained dolphins killed 2 Russian frogmen who were putting limpet mines on a USA cargo ship in Cam Ranh bay in Vietnam. After that, Russian PDSS frogmen were trained to fight back against trained dolphins, and in an incident on the coast of Nicaragua PDSS frogmen killed trained anti-frogman dolphins. Arrival of underwater rifles and pistols seems to make the trained animal threat less.
Animals, unlike ROVs etc., need to be fed and kept in training whether they are needed at work or not, and cannot be laid aside in a storeroom until needed.
A ROV, as well as searching, could be equipped to arrest or attack divers on command, but with their technology as it is could not attack several targets one after another as quickly as a marine mammal. A surface-only ROV would need a long-range weapon to be effective against deeply submerged suspect divers.Siebe Gorman had a policy in Great Britain until around 1956 of keeping prices of aqualungs too high for most civilians to afford; legal restrictions on exporting currency stopped people from importing cheaper foreign aqualungs. See Timeline of underwater technology#Public interest in scuba diving takes off for how this barrier broke down, starting with British sport divers making their own aqualungs from ex-RAF cylinders and converted Calor gas regulators.
The Subskimmer, which is useful for covert underwater penetration, took decades to develop and passed through at least three firms and is still too expensive for sport divers and sport diving centers. This may be due to interference from Ministries. Or it could have been a commercial decision: the market for sports use was judged to be too small.
Siebe Gorman consistently refused to sell rebreathers to the civilian public. Mixture rebreather development was kept away from the public eye and the sport scuba trade until the end of the Cold War in 1991. As a result, when North Sea Oil exploration started in the 1960s, the oil drilling firms needing deep-dive work had to develop nitrox diving techniques independently, from concept up, without using the Royal Navy's know-how; and then the Navy revealed that they had used nitrox diving (which the Navy called "mixture") before 1945.
In the U.S., military rebreathers were not marketed to the public primarily due to cost and attendant legal liability issues. Legal issues still tend to discourage the development and sale of the rebreather in the U.S., though acceptance and use is growing. The U.S. military has not tried to stop sales of rebreathers to the public in the U.S., as it has realized that recreational SCUBA has now exceeded earlier military SCUBA in quality, and hopes that a similar increase in quality and decrease in price will come from commercial-off-the-shelf rebreather equipment.
New technology now exists where underwater speaker systems can be deployed around the designated area(s). This array of speaker systems can be programmed to send high powered frequencies which then blasts powerful ‘disruption’ signals into the water. The frequencies have a maximum disorientation effect on the diver(s), which induce discomfort or panic causing them to leave the area or surface for interception. In cases where the divers remain in the water, the frequencies are likely to have a continued adverse effect which could cause sickness and confusion.
For sport divers and similar who have no means of covert entry, one method is merely to try to stop all divers from reaching water, or stopping them from using boats, in some particular place or area. Such a bylaw may be called for by the military to keep sport divers away from secret underwater sites, or by inshore fishermen to stop alleged poaching of shellfish.
The U.S. has made many such regulations to protect such infrastructures as power plant and nuclear plant water intakes and discharges, bridge foundations, harbor and pier installations, and naval facilities.
The Socialist Federal Republic of Yugoslavia (until it broke up) forbade all sport diving except a few Government-controlled groups, and required official permission for each campaign of archaeological or scientific diving.