A sea-king (sækonungr) in the Norse sagas is a title given to a powerful Viking chieftain.
They could be kings of Sweden (such as Yngvi and Jorund), or sons of kings, such as Refil, but they could also be men "without roof" like Hjörvard the Ylfing; such men without roof could be so powerful that they could subdue a country and make themselves kings. Two examples are Sölve who killed the Swedish king Östen, and Haki who killed the Swedish king Hugleik. However, in both cases they ultimately lost due to lack of popular support.
References
Sea-King Wikipedia(Text) CC BY-SA