It all depends on what you think you need and when. The life of an average Digital Camera isn't too great these days. All the ups, outs, add-ons and extras will not help your photography. They will present you with more options and flexibility in what exposures you do take. Above water I am a Nikon shooter and carry a D3 with a D200 back-up and spend big dollars on lenses to get exactly what I want -- This is because I know what the heck I'm doing and I can leverage the tools Nikon provides.
Underwater is totally different -- I am a pretty decent photographer but I am not experienced enough nor am I knowledgeable enough to get much use out of all the add-ons and wet lens adaptors, etc. The Canon housing is just fine for my needs. Someday, if I grow enough that I can out-shoot the Canon WP-DC21 housing -- I'll probably buy something for a newer model camera that has more features than the G9.
The G9 + Canon OEM housing lets you use 100% of the features of the camera. It requires you two hand a few things, I am not sure how I will like that underwater. The G9 allows me to shoot RAW + JPG and that is very important. Whitebalance becomes far less important with RAW as an option. I have tried shooting with both Nikon and Canon JPG and they do funny things to the images. Nikon calls it Adaptive Dynamic Range (ADR.) Sometimes I love an ADR image and I then use it as a model to post-process my RAW. Usually, I eyeball it in lightroom and am very happy with the result.
The things about the G9 that made me think it'd be a great UW camera -- beyond the amazing shots ScubaBoard members like Adrian share with us are:
1) a fairly wide aperture (since using the camera tho, I am a bit disappointed 2.8 goes away fast in the zoom range and I'm used to my Nikkor 14-24 f2.8 and 70-200 f2.8 maintaining those apertures THROUGH THE RANGE OF THE ZOOM.)
2) RAW
3) A fairly cheap housing that'll work with my Inon Z-240 (Mk2)
4) SDHC support for HUGE cards to let me take bazillions of shots
5) Aftermarket 1700 mAh batteries instead of the stock 700 mAh for longer shooting times.
Yes, the Canon OEM housing isn't idea for the Inon but it'll work. You put the Inon UV pass filter over the flash and duolock the optical cable in place. This is every single bit as effective as the Patima's fancy optical cable mount.
I do not know about the FIT wet-lens housing adaptors but I know the macro on the G9 should let me get fairly close to things I want to photograph and the range on the zoom is fairly good (from the 30mm to 200mm range?) If I was willing to risk my D3, D200 or D70 (which is now a big long in the tooth at 6mpx for me) I'd always be using my 12-24 or my 14-24 and maybe the 50N or some 70mm lens. Getting the longer range on the zoom is great for framing and versatility. I've felt the compact cameras offer more flexibility UW than an SLR or dSLR. The G9 really does function well in fully manual mode. Its not photographer-centric and doesn't follow my pre-shooting workflow (ISO, meter, aperture, shutter, wb, EV, shoot) but rather its a bit more Fisher-Price with its kind of shutter, aperture displayed as variables to change and EV, WB on other buttons and ISO on a dial. I'd just leave ISO at 80 unless you can't get the shot and then step it up, try a shot etc till you get it.
Again, I haven't hit a wall with my UW stuff yet that says I need these wet lenses. Someday I hope to get there. Until then, the Canon housing is great -- its more likely there will be a G15 before I can outshoot the G9.