I'm no pro and am stilling learning but after doing research I bought a Pentax K10D, 10 mega pixel just before Christmas and just before Pentax came up with a new updated model. For the body I paid roughly $600.00. Best "bang for the buck". I also got a Pentax 100mm Macro which was just as expensive as the body but well worth it. That is my main/only lens when shooting into the tank.
One other very, very important piece of photographic equipment is a good tripod - a real[u]must[u].
To do research on any camera that you may be considering, go here:
http://www.dpreview.com/
HTH
Cheers