I've tried three different approaches - Ewa Marine bag for a DSLR (Canon 10D), dedicated underwater p&s (Pentax W30), and p&S shoot in a hard shell housing (Canon G9). Each has their limitations.
I've always had problems with the EWA marine bag leaking. Not a lot and not very fast, but enough to make me nervous and only use it with my backup camera. I see it as a better tool for when you are dorking about on the water and not actually going under the surface. It's cumbersome as well, so I don't do anything but point and shoot while using it - no zooming, no adjusting parameters. On the positive side, it gives me much better picture quality than the P&S cameras.
See more shots taken with the 10D in the Ewa Marine housing
here.
The Pentax W30 was disappointing to me for it's mediocre picture quality and lousy controls. It gets the job done, but I don't enjoy using it and I'm not impressed with the results. On the other hand, it's the only one that is really easy to use and adjust in the water. We had some electronics problems (wouldn't stay powered on) with ours, but I haven't seen a lot of reports of similar problems and we eventually got it fixed right.
See more Pentax W30 shots
here.
The G9 is considered to be one of the better point and shoot cameras. That doesn't say much for the p&s world. It's a clear step up from the W30 in IQ and controls, but the gap between them is much narrower than the gap between the G9 and my 5 year old 10D.
The housing for the G9 makes it even more waterproof than the W30 but it also makes it bulky. You can control all of the controls, but it is much harder because you have to see the everything through the clear plastic housing. It renders the viewfinder (of marginal use to begin with) useless and partially obstructs the flash as well. One other issue that I discovered is that the camera gets extremely hot inside the shell on a hot, sunny day. It was painful to touch when I opened the shell.
See other G9 shots in the Canon waterproof housing
here.