I'm hesitant to offer up specifics for two reasons:
1. When it comes to scuba photography I'm a novice at best
2. I hate to stifle any creativity
My approach was to read up on the very basics. Long on generalites, short on specifics. Then get down there and shoot, shoot, shoot. Check your work, see what you like, then do some advanced reading. You might find that what you've done that you liked doesn't conform to the 'usual' rules. Much of it will though. Things like rule of thirds, specificity of subject, and directionality are almost axiomatic. But it's neat when you get a shot that breaks all the rules that really works. If you're starting out by pedantically following the rules you might miss out on that which you could have discovered organically. That'd be a loss.
One caveat - I was SLRing back to high school before I took the dive with my Rebel XTi. If you're not at a point where you can fiddle with f stops and exposure durations then discount my thoughts accordingly. The wildest composition can be ruined by blown highlights and overexposed whites.
m.