A sea-king (sækonungr) in the Norse sagas is generally a title given to a powerful Viking chieftain, even though the term sea-king may sometimes predate the Viking age.
The Orkneyinga saga contains the earliest reference to sea-kings. There the original line of 'kings' of Kvenland (present-day Finland) ends with the father of Gor Thorrasson 'Sea King'.
Sea-kings could also be independent or noble Norwegian and Danish chieftains, and also kings of Sweden (such as Yngvi and Jorund), or sons of kings, such as Refil. However, 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.
List of sea-kings as given in the Nafnaþulur 1-5
Ale the Strong Asmund Atal Ati Atli Audmund Beimi Beimuni Beiti Budli Byrvil Ekkil Endill Frodi Eynef Gaupi Gæir Gauti|Geat Gautrek Geitir Gestil Gjuki Glammi Gor Gudmund Gylfi Hagbard Haki Half Harek Heiti Hemlir Hiorolf Hjalmar Hnefi Hogni Homar Horvi Hraudnir Hraudung Hun Hunding Hviting Hæmir Iorek Kilmund Leifi Longhorn Lyngvi Mævi Mævil Meiti Moir Mysing Nori Næfil Ræfil Randver Rakni Reifnir Rer Rodi Rokkvi Skefil Skekkil Solsi Solvi Sorvi Sveidi Teiti Thvinnil Vandil Vinnil Virfil Yngvi
References
Names of Sea-Kings ("Heiti Sækonunga") by Björn Sigfússon in Modern Philology, Vol. 32, No. 2 (Nov., 1934), pp. 125-142