Back in the dark ages of film cameras I noted that my Nikon SLR just gathered dust on the shelf, because I was too lazy to bring it with me most of the time. Instead, almost all of my 35mm photos were from a compact Olympus Stylus non-zoom 35mm camera.
So when I decided to go digital, I chose the Olympus Stylus 400 and the Olympus housing that goes with it. Nice and compact camera both above and below water.
The main drawback of any point and shoot camera is the very weak flash, which usually limits you to available light shots for all except macro.
Knowing what I know now, I would not have gone with the Olympus Stylus, because it lacks manual white balance and the ability to manually preset the focus distance. There are some other small, relatively inexpensive cameras that do have manual white balance and manual focus in the Sony, Canon or Pentax lines that would have done better.
I wouldn't sweat getting the highest megapixel camera by any means. For casual underwater shots using available light, your photos will be limited by things like getting good focus, color distortions, and blurring due to camera motion long before any megapixels beyond 4 or 5 become an issue.