No matter how good your eyesight and how clear the water, the underwater world looks hazy, distorted and sometimes downright murky to humans. This is because when you're under water, the cornea isn't nearly as good at bringing light to a focus as it is on land. Fish have special adaptations that allow them to see underwater and at great depths, and some fish even see in color. Because of the way light is refracted in water, fish have a wide "cone of vision" of about 83 degrees. It is like looking up from the base of an imaginary funnel. As a fish goes deeper, his window to the outside world grows. Even more interesting, if the water surface is relatively smooth, a fish can look up and see a mirror-like image of the bottom. This allows him to be aware of either prey or predator beneath him.
To learn more, read Understanding Your Fish's Senses