I used a Cannon S200 Elph with the corresponding underwater case and was very satisfied with my pictures. I should note that I'm a snorkler, not a diver, and was just looking for vacation snapshots (as opposed to professional results).
When you take pictures underwater (as others have noted) you will lose the red portion of the spectrum. I used Adobe Photoshop LE (free software that came with my camera) to adjust the photos and it really made a difference. I just used the "Auto Levels" feature (I know that Photoshop experts will scoff at Auto Levels but they worked for me!).
If you want to check out my pictures (both underwater and not) check out my web site and click on the Belize link on the left. Underwater pictures are (mostly) on Days 2, 3 and 6. You will need to go past the first page of pictures to see most of the underwater ones.
On one of the pages I even have a "before" and "after" picture showing the image quality before using Photoshop and the image after to give you an idea of the difference.
The hardest thing was taking pictures of fish. You have to get pretty close to the fish to get a good shot (at least with my camera!) and fish, for some reason, don't want to let you get too close. Plus, most consumer digital cameras have a delay between pressing the shutter button and when the shot actually takes so the fish would often swim out of the frame before the picture actually took.
My best advice for whatever camera you get is that you practice, practice, practice with it before you go on vacation. Make sure you are comfortable with getting the camera in and out of the housing and that you know all the controls. Also, when you get to Belize, don't hesitate to practice some more off your hotel's pier before you take the camera out to an important snorkal/dive site. You don't want to waste precious time learning your camera when you should be enjoying your vacation.
Oh, yeah. Make sure your camera has fresh batteries before you go on a snorkal/dive trip. Nothing sucks worse (well, nothing in this context) than running out of juice in the middle of a dive. So buy at least two batteries and alternate them.
Also note that the S200 is an older camera and is only 2 Megapixels. Plus, the images on the web site are reduced in size and quality to improve loading. My Web Page.