what kinds of cameras are you trying to compare? differences b/w DSLR's or b/w P&S's, or DSLR vs P&S? I will second alex's point that you'll find more variability in photographer skill than the camera itself.
its certainly true that a camera is just a tool and the skill to wield makes all the difference. sure a dslr with a decent optic & port etc has a superior ability to a P&S. but other factors will completely eclipse the choice of DSLR: lighting & exposure skill, optics (lens + port), water clarity, #strobes etc.
I am a firm believer that the lens you put on your camera is far more important than the camera itself. A crummy lens on a D3x is inferior to a good lens on a D70. There is fantastic info in these forums on the best lenses for UW use.
so by all means compare cameras, just be aware of the more significant factors. cheers,
chris