first question, whats ur budget.
and judging from your D90 budget consideration,
you can consider
Canon: 500D, 50D (a few hundreds more)
Nikon: D5000, love the multi angel live view.
personally i like my D90 but i will like it even more if theres higher ISO
and if i want to upgrade, i will go D700 if i have spare cash to spend.
if u are serious about diving taking picture etc, get a high ISO performance like the D700. that if u have the spare cash to spend.
and i went for nikon D90 because it can be consider mid-range and i find the rest i mentioned, other than D700, rather plasticky.
and next question, how often do you actually have the time/place to dive? is it worth it? thats another question.
my vote goes to D90(got HD video recording)/50D(no video but higer ISO and pixel) for normal using, its good enough.
and video recording do comes in handy.