If you can see 80 to 100 feet underwater, your subject should be no more than 20-25 feet away. If you'll be on an escorted dive, inform the divemaster that you'll be taking photographs. He or she can point out the best subjects and will wait a few extra seconds for pictures before moving on. Move in as close as your equipment will allow. Many underwater subjects are small and look best when they fill the viewfinder.
Wrecks can make dramatic subjects.
Unless you have a super wide angle lens, pick portions that have attractive structures, and consider placing another diver in the picture for scale. Don't photograph your bubbles, or those of another diver, unless you want them there for drama. Time your shutter release for when you're inhaling. Be patient and keep your subject in the viewfinder. If the subject is hiding in the coral, just be quiet and wait for it to appear.