A Sea King
was the Viking
name for a powerful pirate
They could be actual kings of Sweden (such as Yngvi and Jorund), Denmark or a Norwegian kingdom, and sons of kings, such as Refil, but they could also be men without roof like Hjörvard the Ylfing.
In the Norse sagas, 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.
List of sea-kings as given in the Nafnaþulur 1-5