The best tip I can give anyone interested in underwater photography is to realize the limitations of your camera equipment. Getting your photo's to resemble what you see in your viewfinder is not quite as simple as it sounds. Decide what you want to shoot and purchase your equipment accordingly. My interest's are primarily the wrecks themselves which means I shoot wide angle almost exclusively. The trade-off has been countless missed opportunities to shoot sharks, rays, and other sea critters.

There are camera systems available that enable you to change lenses underwater which greatly expands your opportunities on every dive, but they can be a little pricey. You don't necessarily have to invest a lot of money to get professional looking photo's. I have a friend who's taken some great macro shots of clownfish fit for a magazine cover using an inexpensive Ikelite housing for use with disposable camera's.