For the past 10 years, I have gone to Maui twice each year. I spend about three months there each year. I have experimented with underwater photography for several years, now. I have owned simple Canon and Nikon point & shoots to Sea & Sea eqipement with an external strobe. I can make two definite recomentdations. You absolutely must use an external strobe. And, if you think it is powerful enough, get one size bigger. Adequate light is everything under water, especially past 8 feet. You simply can't get all of the colors without it. Second, make sure you have a zoom lens of some sort. You can only get so close to underwater creatures without their wanting to scoot away from you (with the exception of sea turtles). I have also often considered buying a waist weight belt to help stabilize the body bouency problem you can't avoid. And, finally, don't waste time going under if the water isn't ceystle clear and calm. Otherwise, You'll be very disapointed. Unless it is an unusually calm day, I generally don't shoot under water after about 9 AM.
As for the Canon G9, it is a great camera. And, the under water case is very reasonable, considering the DSLR alternative. But, be prepared to also attach a powerful external strobe.