Well, you didn't mention what your budget is, so we could run the gamut of low, mid and pro range DSLR's and their prices.
Maybe try a Go Pro Hero 2 HD? They come with a waterproof housings and can shoot stills, video, even time lapse. They're usually about $300-$400 depending on the accessories it comes with. I think they make aniother special housing for true underwater, but I think it can just go a lot deeper. I don't own one, so I could be wrong, but I've been looking into get a Hero 2 HD for a few months now, just for doing more outdoor stuff that I wouldn't want to put my Nikons though
If price isn't an issue, as Rock said, most underwater housings are pretty expensive, but available for most DSLR's. You'll probably have the best luck finding housings for Canon's and Nikons. Also, there are several underwater "cases" for DSLR's too, which are essentially gloirified ziplock bags. They're cheap, usually between $100-$200, but they don't go as deep and are not as reliable. I just looked up some underwater housings for my Nikon D7000, and I haven't seen one for less than $1400 yet.
There are some pretty great websites that also have a lot of great information, like:
Underwater Photography Guide
I guess you should figure out what kind of DSLR you want to go with first. Do you plan to shoot both video and stills? How much are you willing to spend? What kind of lenses do you plan to use underwater? Etc, etc.