Taking photos underwater adds an entirely new dimension to the enjoyment you receive from diving. This guide is geared toward using a point and shoot camera, which is what most of us start out with. Unless you have a waterproof camera rated for a certain depth, you're going to need a waterproof housing, which you can buy from your camera's manufacturer. These housings are designed to fit your camera perfectly and protect it from pressure and water.
Before you get started, get to know your camera. Learn where all the controls are, and spend plenty of time playing with it topside, preferably in a dimly lit room. Test the camera's range with and without macro, and take photos of small objects close up and see how they turn out. Once you've done this, you'll be better prepared to take underwater photos that turn out well.
On Your First Photo Dive
First and foremost, be sure you've got a handle on buoyancy control. You don't want to crash into a reef, and you do not want to float up to the surface unintentionally! Taking underwater photos can be quite distracting. Remember, you still need to pay close attention to your surroundings and continue monitoring your dive computer and other equipment as you do when you're not taking photos.
Second, you're going to need flash to ensure your shots are colorful, unless you're photographing objects further than 3-4 feet from you. Make sure your flash is set to forced flash instead of auto flash. Without flash, everything you photograph will look mostly blue. If you're within 2-3 feet of your subject, turn on the internal flash and use auto-white balance. Avoid using the "cloudy" setting since that can cause your photos to take on a reddish-orange cast.
Positioning Yourself for Photos
Remember what you learned while testing your camera's macro mode topside. Most cameras' macro modes work well with shots taken 1-2 inches to about 2 feet from the subject. For right now, keep your camera zoomed out to increase your ability to focus well, and get as close as possible to your subjects. Get down low, at eye-level, with the subject facing you if possible; and if you're photographing a fish or another animal with distinct eyes, focus on the eyes. Remember that the closer to your subject you are, the better your photo will be. Try to "fill the frame" with the object or animal you're photographing.
Taking Photos Without Flash
If your subject is further than 3 feet from you, you're going to want to take your photo without flash, using ambient light only. To get better color, you'll either need to do a manual white balance (check your camera's manual to find out how) or use underwater mode. If you take photos of distant subjects while using your flash, you'll end up with backscatter unless the water you're diving in is exceptionally clear.
Ways to Improve Your Underwater Photography Skills
It stands to reason that a brand new underwater photographer won't want to spend a lot of money on camera gear, and that's fine. Enjoy your point and shoot for now. If your camera's housing came with a flash diffuser, be sure to use it, and leave your camera on automatic mode at first. Keep practicing composition and learning how to use light.
You can greatly enhance the quality of your underwater photographs by purchasing a camera with more controls, and getting an external strobe is the best way to improve your photos.
The bottom line? The more you practice, the better your photos will get as time passes. Emulate famous photographers by taking pictures of everything - you'll be surprised by the way even the humblest subjects take on new life when given a chance to shine.