SS Poseidon was 1968-1969 the name of a steam cargo ship built in 1941 for the British Government and originally named Empire Ballard.
ST Poseidon was 1973-1976 the name of a steam tug built in 1941 for the British Government and originally named Empire Fir.
HMS Poseidon was a Royal Navy Parthian-class submarine, launched in 1929 and sunk in 1931.
USS Poseidon was a US Navy Achelous class repair ship, built in 1944 and sold off in 1961.
In fiction
Poseidon, the subject of the 1969 novel The Poseidon Adventure as well as the films based on that novel: The Poseidon Adventure (1972), Beyond the Poseidon Adventure (1979), The Poseidon Adventure (2005), and Poseidon (2006).
USS Poseidon, a US Navy attack submarine in the 2005 film Phantom Below.