King Neptune was a powerful ancient sorcerer who acted as the protector of the seas in his own time. He's considered to be a predecessor to Aquaman. Whether he was affiliated to the deity Neptune/Poseidon was not explained in his backstory.
Neptune is mortal and suffered the fate of all mortals, aging and dying. Prior to his death, the hero had predicted a undersea volcano exploding in the distant future. The explosion would result in an ecological disaster which would end life as he knew it. Miserable at not being alive to stop this event when the time came, Neptune performed a spell. When the time came, a future adventurer would be lured to his grave and transformed to a new incarnation of himself. His mission was to stop the events from occurring.
The spell worked and centuries later affected Captain Crane, an eccentric marine archaeologist. He witnessed the volcanic explosion and the tsunami resulting from it. But he used his powers to disperse the giant wave before it reached coastal areas. Then the spell ended and Crane was returned to his normal state.
Neptune had previously used his powers to transform a fish to a winged sea-horse (his steed) and disrupt shipping in the area of the volcano. Aqualad assumed his trident was the source of his powers and attempted to take it away. Neptune retaliated by turning Garth into a giant-crab. Whether he had other abilities or if the spell could be invoked again is unknown.