Photography articles commonly focus on the technical side of photography--cameras, lenses, lighting, etc. While it is necessary to understand at some level how your camera works, I've have always believed that an understanding of composition, animal behavior, and how the photographer's behavior influences the creation of an image are more important to understand. This is especially true when that once-in-a-lifetime photo opportunity swims by, since it is natural to lose your concentration in the excitement of the moment. To maintain your focus, all you have to do is to remember the photographer's mantra. The chant is simple: get down, get close, shoot up. If you remember and use these three simple phrases, the quality of your images will improve dramatically.
Get Down. This concept is contrary to what is taught in scuba classes and your divemaster's instructions to maintain neutral buoyancy and don't touch the reef. Images taken above a subject and looking down are often boring because the subject generally blends into the background and is hard to locate. Better images are created when you get level with or below your subject. This position allows you to obtain a bold perspective on your subject, and enables you to isolate your subject from its background and create images where the subject is not lost in the clutter of the background.
Another reason is not obvious but very important--getting down gives you a better opportunity to find subjects! Most critters are superbly camouflaged when viewed from above, and are often easily located when viewed from the side.
Of course, you do not want to land on the reef and crush your potential subject. Working along walls, overhangs or large rocks allows you to get down relative to your subject without harming the reef. Patch or low-profile reefs are more difficult, but you can normally find a bit of sand to kneel or lie in to get a level perspective on your subject.
Get Close. Sometimes, as an underwater photographer, it is hard to accept that water is the enemy. Water supports all of the plankton, sand, schmutz and sparkly bits we photographers refer to as backscatter. The relationship is simple: the more water you have between your lens and subject, the less sharp you image will be and the more backscatter you will experience. When you get close to your subject you get clearer, sharper images. This is why photographers rarely use "normal" or telephoto lenses underwater--they use close-focusing macro and wide (or ultra wide) angle lenses. These lenses get the water out of the image.
Getting close has another advantage; it allows you to fill the frame with your subject. It is generally a bad idea to expect your audience to hunt for the subject in your image. If you have ever looked an image and asked, "What am I looking at?" you understand what I am talking about. Your primary subject should "pop" out of your image. Your audience may not know the name of the critter they are looking at, but they should easily identify the subject. Getting close to your subject makes composition that much easier.
Shoot Up. Shooting up allows you the ultimate control in creating images that explode off the page or screen. By shooting up you can create a jet-black background, a blue or green background, or a background dominated by the sun and its rays. It's all about the juxtaposition of your subject, camera and the sun. Use a small aperture and point your lens away from the sun and the background goes black. Select the appropriate aperture, and an aim point near the sun, and the water is bright and shows off its color. Aim right at the sun and the background is overexposed and the subject really stands out. Even if you cannot obtain a clear water background, you can still isolate your subject by framing it against a more distant and out-of-focus background.
What is not so obvious in shooting up is that it creates a perspective that encourages your subject to show off its personality. Good photography captures a moment. That special expression, position of a fin, or twinkle in the eyes often separates a good image from a great one. Shooting up allows those great expressions of what it means to be that fish or seal to be clearly recorded and not lost in the clutter of the background.
Whenever you encounter that ordinary or extraordinary subject, remember the mantra. Get down, get close, shoot up. This simple phrase often allows you to focus on important elements of your image, and capitalize on it before the moment is lost.