A subject can be dark, light or even reflective will change how an impressions are recorded on the film. By using the f-stop and trying three different setting, the lighting of a subject can be obtained. The background water is always blue or dark. If one angles the camera up toward the surface and the sun a contrast in the background color can be achieved. Be careful in shallow water and point away from the sun where white sparkles on the surface can result in white streaks on images.
Subject selection is crucial in getting a quality picture.
Fish that are moving tend to blur or the background smears. Try to photograph a setting with a fish or sponge as the subject. Have a diver in the background to illustrate depth of the surrounding water. A fish swimming slowly at three feet away with no close background or diver will seem flat. Add dimension to the portrait.