One thing that hasn't been mentioned is the buoyancy that the housing provides. I got a housing for my Canon Sureshot S70 (not made anymore - it must be 5 years old and superceeded now) and use it for some commercial marine jobs. The shots are fine, but it is SO SLOW. The shutter lag is terrible. For things that aren't moving fast it is OK, but underwater shots of outboard motors and propellers it is a pain, having to shoot over an dover, trying to anticipate when the shutter will fire - the lag must be more than 0.5 secs and sometimes closer to 1 sec. A speedboat has disappeared out of shot in that time!
When trying to just go over the side with it for quick shots (out doing a feature on charter angling mid channel for sharks, I dived overside to get shots of the anglers landing a shark but shot from the water) with no fins, mask or snorkel, the buoyancy of the housing was more than I could overcome for any length of time. Canon will tell you how many of their propietary weights are required to neutralise the buoyancy. With no weights the housing with camera inside floats.
I am upgrading this set up to the new G10, when it is available, and with the dedicated housing for it - I am that pleased with the results (that old S70 of mine shoots RAW, so is good for even A3, full bleed, DPS's)
You don't have to use the dedicated weights (the new G10 and housing requires 4 of their weights) just get some lead sheet from a roofer, you can cut it with tin snips, and make some of your own - there is a tripod thread on the base of the housing, which I doubt you will need, just bolt the lead in place there and trial and error it in the bath/kitchen sink - when you have just enough so it sinks very slowly, you are close enough.
Water clarity will be the limiting factor to your pictures - sand/silt or even plankton bloom can make thngs very difficult.