Genos, if he hadn't thrown the battle early on, perhaps; Lightspeed Flash is Sonic but faster, and Sonic would have defeated the un-empowered Sea King if he had his equipment; Tatsumaki, Watchdog man, and Atomic Samurai probably have enough offensive power to kill him before the regeneration kicks in (no such confidence in Metal Knight and Child Emperor's arsenals though); Zombieman would just outlive him to death; and of course there's no need to mention King, the strongest hero alive.

King can't fight the Sea King, that would be beneath him. King only fights when a REAL challenge comes across him. The Sea King would just be a waste of time.