The problem with underwater photography is light. If you are more than 10 feet deep, you have lost all the reds, and below 30-40 feet, you can't really take a good photo without strobes or lights of some type. I use a Canon G9 in the Canon underwater housing. There is a light diffuser for the onboard strobe and that works for most photos, but anything more than 4 feet from the camera will not get illuminated.
If you want to do serious underwater photography, you have to get some serious underwater strobes. Ikelite makes the AF35 which will attach to the Canon case. I tried one and found it to be okay. It triggers off of a light sensor. When the camera's onboard flash fires, it trips the strobe. Mostly okay, but not 100% reliable.
Nothing beats a good wired in strobe, though. My main underwater system is a 30D in the Ikelite case with two DS-125 strobes. The strobes are wired TTL and give great light. The whole setup costs more than the camera and lens, so not for the faint of wallet, around $4,500.00 for everything. But the difference in the images is night and day.
As one guy told me once, it only costs a little more to go first class. lol