Definitely the D90 from those two but you can also look at the Rebel XSi.
Actually, I stand by the above comment but have a couple of questions that you really should answer before somebody can tell you which may be better for you.
1. Is your dive count accurate 0-24? If so, I would suggest you take some time to dive, hone your skills etc. before getting too carried away with large camera purchases. Buoyancy is paramount in underwater photography not only for your safety but also that of the animals and reef. Task loading is also a large "potential issue" with new divers.
2. Are you a photographer topside? Are you fluent with SLR cameras and their technology?
3. What are you hoping to get from the pictures? Are you looking for something you can blow up and sell or just memories for yourself?
If you are really a new diver and are not too experienced a photographer, I would likely suggest you go with a decent P&S (like a Canon G9 with Canon housing) rather than DSLR. The housing alone for the D90 likely costs 2 or 3 times what the G9 AND housing together would cost.