Fathom(1983)

Titans have imprisoned Neptina, Neptune's daughter! They've shattered his magical Trident and scattered its pieces far and wide. Without his trident, Neptune is powerless. Proteus, a member of Neptune's Court, longs to free the lovely mermaid. He sets out to find the missing pieces of the Trident. He roams land and sea, changing from a dolphin to a seagull and back again in order to deceive the mermaid's captors. Deadly trials confront Proteus -- octopuses, tangled beds of kelp, screaming flocks of black birds, even erupting volcanoes! But he fights on valiantly!