Submariner

Submariner is one of ZX Spectrum's "3-D" games, in that it is played and viewed from an isometric overhead angle. Players essentially take role of a scuba diver in an undersea story to search for the "evil Nigel Lobb." So no, the game isn't about the classic Marvel Comics character. No real detail is initially given for why players are out to get Nigel Lobb, aside from saving mermaids from him. However, he is noted to be in control of various deep-sea lifeforms, including but not limited to jellyfish, giant crabs, "poisonous" air bubbles, including said mermaids.