Where is the oxygen again in the water. They can't have lungs but instead use their gils to get O2 out of the water to live. We can't do that. In very shallow waters, animals that crawl on the bottom can still get plants and other fish. Plenty of food, because of the plants.
Food is mainly other fish, little to no plant life.
Strong bodies to protect against heavy water. Fewer things can afford to just stay still and wait but must go out hunting for food. No eyes for seeing, other ways of finding their prey.