My wife and I just returned from Puerto Vallarta Mexico. During our stay we went snorkeling at Los Arcos south of Puerto Vallarta. Our guide took us on a short 1 hour trip. We arrived at Los Arcos and the guide began feeding tropical fish with breadcrumbs. A huge school of angel fish appeared and I jumped in the water with an underwater Sealife 35MM camera. It was really weird being in the middle of this huge school of fish, I could touch some of them and felt them swimming by. At any rate I took a bunch of photos of which only 3 seemed reasonably decent. The instructions for the Sealife camera stated that I should take photos of schools of fish 3-5 feet away with the flash always on. I did this using 100 speed film. Sealife recommends 200-400 film since there is not as much light in underwater photography. I found that in most of my 24 photos that the fish were too close or too fast moving-very frustrating.
I returned to the boat and found I could take better photos with a conventional digital camera of the fish feeding at the surface of the water though glare was a problem. So long as the guide kept feeding the fish-the fish hung around the boat. In the Pacific Ocean the water is not as clear as the Caribbean but almost in certain places. According to a scuba instructor at Club Med the Indian Ocean has the clearest water and the best chances for underwater photography. Even the smallest particles in the water can defeat a photographers efforts.
If you shoot digital you have an advantage over film in that you can review the photos immediately and go back and re shoot. Underwater housing are available for many point and shoots as well as DSLR cameras. Use the electronic flash for your camera otherwise the photos will be very blue. Only exception is very clear water and close to the surface. If you have camera but no underwater housing just shoot the fish at the surface from the boat when the guide starts feeding the fish. A polarizer might be helpful though it will necessitate 1&1/2 stops or more additional exposure. Good luck!