A waterproof case is meant to function down to 30 meters or something like that, so in a way it is overkill.
I rented a digital camera in a case for some of the dives I did in Australia a few years ago. It did give me great images. A pro might opt for a special purpose underwater camera for optimal image quality, but thats a whole other world.
I also rented a cheap Sea & Sea underwater camera for one dive and image quality was quite dissapointing, so a good digital camera in a case is better than a cheap underwater camera with crappy lenses.
The purpose built waterproof bags (the ones you can take pictures from) seem to work fine for surface pictures (they are quite waterproof), but do not handle preasure at all. I don't know how they affect image quality or how durable the bags are.
Certainly with either, you can go for a swim without worrying about the camera (unless if floats off or is swallowed by a whale).
I wonder if you can find somewhere to rent a camera and case to test one out? Maybe ask at a local dive shop??
