Hi,
I run a Canon G-9 in the stock Canon Case. It's great. Yes, the built-in flash is crap for underwater. Don't despair, you have a phenomenal camera that will grow with you and is the best of the point and shoots.
I bought a "tray" which really means mounting bracket from a small Seattle company.
Now, I'm gonna give you the bad news about dive photography before I tell you what strobe I run.
1) All the players want your money. Period. They will try very hard to sell you the toy camera in the LDS and the over-priced brand new strobe. Sea&Sea's answer when I asked for parts support was that a strobe I had was "non-repairable" and "they didn't provide support".... which is code for "you're screwed".
2) The beauty of a point & shoot is it's TTL interface. Unless you enjoy math and light calculations or what to revisit the world of film and frustration.... you want to use your TTL sensor. Canon has a great TTL computer that "figures out" the flash.
3) All dive gear eventually floods. Maybe if you are lucky I'm wrong, but plan on it. Put the amount of money into your gear that you are willing to throw out the window while driving on the freeway.
4) My philosophy is that I bought a used Canon G-9 with a used case, a new o-ring, and new-gel. I bought a USED YS-120 (8 AA battery) strobe... a good mid-range strobe that gives the fish sun-burn.
I bought a synch cable from Sea & Sea... and then I bought a Heinrichs-Weikamp TTL converter. No holes in my camera case and it works just fine.... it "fools" the strobe into thinking it's talking to the computer. It is manageable and settable - decent instructions.
You probably won't find a HW TTL converter in a LDS. You'll honestly be doing good to find someone who understands TTL and knows what a converter is. They would rather sell you a really expensive IkeLight case, strobe and cable.
Ebay is a great place to find a used strobe. I bought mine from a camera collector on Ebay who found it at an estate sale. I think I paid $80 for it. He was happy, I'm happy, and it works fine. I use Eneloop AA NIMh batteries with a MAHA 8-cell charger. It will recharge in 8 hours....
The hole rig is slightly negatively buoyant.
The tray came from Optical Ocean Sales -
Camera Housings, Strobes, Arms, Trays & More! - Optical Ocean Sales Underwater Photo - 800-359-1295!
A small Seattle based company, he's on SB as well.
Lastly, I'm not a guru..... there are plenty of those on here.... I'm like you I want to take dive pictures. I do have a land-photography background though and knew I would outgrow a SeaLife. For the record, SeaLife is a good camera for someone who wants to take snapshots. It is not a photography grade camera.
Oh, your G10 probably does HD video. Videos is even easier then photography so spin the dial and give it a whirl next time you are in the water. If my memory is right the G9 is slightly more advanced then the G10, but they are both great cameras that have a range of options for learning.
If you haven't done so, set aside some time in the pool or your local dive environment to go out and play with the camera. Practice makes perfect. Find a dive photo club if there is one in your area. You will learn alot from other divers who can explain the finer points of getting it right so you can get great shots on your next trip.