I had CPC Jaws, it was an action game.

Pure videogame logic. You had the tiniest submarine and you had to explore underwater caves full of killer starfish and other unspecified marine creatures. The goal was to fish out parts of the ultimate anti-shark gun and use it on the shark.

Said shark patrolled the area just below the surface. Not sure how big the submarine was supposed to be, but comparing sizes, I think that shark must have been a freaking megalodon.