Let's assume the planet has only one half the mass of the moon and is covered by shallow ocean. To eliminate water pressure at extreme depths, assume the ocean is also quite shallow, with a maximum depth of 100 meters.
How would critters living in this ocean be affected? How would life evolve? For the purpose of the question, the planet somehow has an earth-like atmosphere so the alien fish can breathe.
Edit: the ocean MUST be no more then 100 meters deep at any given point. Assume we are dealing with very shallow water here.
Edit 2: for the purpose of simplicity, the mass AND volume are exactly 1/2 of the moon.