Here are a few ideas.
You play as a scuba diver looking for the smallest sea creature in the ocean.
You play as a fish that has to eat as much other smaller fishes using the smallest number of moves possible. Propel your fish using a slingshot effect.
You play as a creature that needs to get up to the surface by catching a ride on other sea creatures. Try to reach the surface with the least number of hops or falls.