Sea King is someone who juggles multiple romantic or sexual partners at the same time, usually by being smooth, charming, and evasive. The idea is that the ocean is vast and full of "fish," and the Sea King rules them all. A Sea King usually doesn't commit, keeps things vague, and makes each partner feel "special," while in reality having many others.
Usually it has a derogatory tone, but sometimes playful, like calling someone a "player" or "Casanova."