It depends on what results you want, how adept at photography you are and how much you want to spend.
You will need a camera that shoots RAW. RAW allows you to adjust white balance in post processing without penalty. That ability is a real big deal in underwater photography.
For the best results, you will want a strobe. As you descend, the water column absorbs long wave lengths of light first. Red light is gone along with red colors in just 15'. If you want "normal" colors, you have to have a strobe. But strobes cost $$$. Underwater photography is not cheap even with point and shoots. Still with a good camera and a good housing, you can get decent photos in shallow water, 30' or less.
I use a Canon G11. It shoots quite a bit like an SLR which is easy for me. It also takes very nice top side shots. It makes for a nice compact camera to have along when lugging around an SLR and lenses is not practical.