A few books:
Jim Church's Essential Guide to Composition, 1998
The Underwater Photographer by Martin Edge, 1996 (there's a 2nd edition)
Essentials of Underwater Photography by Robert M. Jackson, 2000
The Art & Technique of Underwater Photography by Mark Webster, 1998
A few tips:
-shoot in manual while underwater (cameras make dumb assumptions for UW shooting)
-know what makes a good image (critical focus, exposure, lighting, composition, negative space, impact)
-visualize images you want to shoot (sketch them so you don't forget)
-know what your camera and lenses can do
-look at your images, fix your most common mistake (repeat as necessary, fixing the next most common mistake)
-make good images with your camera (don't rely on Photoshop to "fix" everything; remember garbage in garbage out)
-look for good negative space _then_ look for subjects on the good negative space
-tell a story with the image