Underwater Camera Bodies
Initially you will want to have an idea of what type camera body you will use. Your choices will be a compact camera (Point and shoot), dSLR (cropped sensor), or dSLR (full frame). See the underwater camera comparison guide for choices. Like I mentioned before, for dSLR users, your lenses are more important than the body you choose.
The dSLR myth:
I just read on a website, that dSLR’s require more knowledge to use than compact cameras. I disagree. A dSLR right out of the box, put on “program mode” will take great photos. You simply have more options than most compact cameras, if you choose to use them. But you don’t have to. Someone experienced taking great photos underwater with a compact camera will often take great photos with a dSLR camera their very first time using it underwater. In any case it is important that the underwater photographer understand how the various camera settings will affect her pictures and even more importantly, she should be very familiar with how her camera works on land before she takes it for a swim.
Combination body + housing
Several companies sell camera bodies and housing's together. Sealife and Sea & Sea are examples. Some people feel better systems can be put together by getting a housing and camera separately. Other people enjoy the convenience of purchasing a housing, camera and strobe all together in one system from one company.
Please review the underwater camera guide for choosing a camera body.