The S100 and G12 you have listed are pretty different camera's. You might think a bit more about your priority's. The g12 is a high end point and shoot with a flip out screen. It's nearly twice the size of the S100 but has more options and bells and whistles. It's often seen as a good 2nd camera for someone used to shooting with an DSLR.
The S100 is more "pocketable" camera that has a rep. For good image quality in a small camera. IIRC it will also shoot in RAW which is a big help if you want to adjust your photos before you print them.
There are also other camera categories for "super zooms" and weatherproof camera's. They've got some that can shoot hd video underwater.
If you want the smallest that takes good pics I'd try the s100. If you want the most control go with the G12. If you are rough on stuff and/or snorkel a lot you might look at a waterproof. Lots of camera's, lot's of Niche's depends on what your priorities are.
I have an earlier version of the G12 the G9 and much prefer it to the smaller P&S camera's I own. it's still pocketable but takes better pics. They are soon coming out with a new larger sensor version called the G1x that I'm going to buy as soon as I can get my hands on one. I have DSLR's and my priority in a P&S is the best image quality in something that's pocketable.
Good luck! .......... Dj
