I bought a canon G11 this year specifically because I am a diver (circa 90 dives, so still lots to learn), and there are a number of waterproof housings around for that camera. In particular, I bought the Canon housing, and it works a treat up to 30 metres. If you go this route, make sure to buy the extra weight set that screws onto the bottom of the case, this gives the camera neutral buoyancy, and if you don't it gets really annoying pulling out of your wrist
Recently back from a live aboard on the Red sea and there were a lot of photographers on that trip with real super-duper equipment, some running to thousands of euros worth. However, there was one diver there with a G9, Ikelite case and external strobe taking amazing shots. I thought I'd made the wrong choice with the Canon case, as there is no external output socket for a stobe (whereas there is on the Ikelite) - but upon returning, I discovered that you can attach an external light sensitive trigger to the Canon case just in front of the internal flash.
An Ikelite housing is likely to cost twice as much as the camera, so I think I'll wait a while. But I will buy an external strobe for the next trip - once you go below 10 metres depth (even in clear seas like the Red sea) you get a real case of light loss, colour loss and shakes set in. The internal flash is useless at distance, and the design of the case blocks the flash on the bottom 3rd of macro shots.
But I'm happy to learn what to do without a strobe first: there is enough to worry about in diving without adding another electronic gadget...
All advice about diving with an underwater housing says that on the first dive (your check dive) of a series, that you take the housing down without the camera in it - just to make sure there are no leaks).
Have a look at Underwater Photography Guide for loads of tips and a good tutorials
(PS: One practical piece of advice I never saw, and wish I had is to never dive with a shorti wet suit and take photos: You'll need protection from all the things you bump into while concentrating on exposure and aperture!)