Underwater prey defense mechanisms?

The solution is to learn something about fish. Look at anchovies and sardines. When they are schooling near the surface, the reflections from the silvering helps them blend in with the sunlight from the water surface. Lots of fish lie on the ocean floor and blend in with their surroundings. It helps both hide them from predators and hide them from unsuspecting prey. Watch some of the excellent programs on Nova, Nature and so on. It's really easy to understand when you can watch it happen.