I agree the size and weight alone, for airline travel these days, of a full SLR rig is daunting. Plus you have to chose what to shoot ahead of time, rather than being able to zoom from WA to macro, and enhance those with wet lenses.
The problem with the canon G cameras is their crippling slow AF lag. Slow in good light, slower in dim light.
The pan lx5 is the best p&s cam for UW today. There is a helpful write up on the 3 top compact cams today, by reef photo:
http://reefphoto.com/kb.php?id=28
For me, the sad thing about these cams is the lack of view finder makes them much less useful above water.
I would love a canon G12 (I had a g2 and g3 and finally gave up), they just don't auto focus anything like as fast as the competition.
I have slim, but not dead hopes the nikon p7100 will cure the AF lag issues of the seriously flawed p7000. The reviews are not in yet.
The mirrorless micro 4/3 cams are interesting. The oly epl2 and epl3 and oly housings are inexpensive, but they limit you to shoot one thing at a time - macro with a $600 macro lens, which focuses slowly and drains the battery on the epl's. None of the oly's come with viewfinders, so you need to plunk down around $200 just for an EVF, which does not show you live view in burst mode, so it point and guess.
The pany m4/3 cam I want is coming out early next year - a rangefinder style camera with built in EVF.
Today, if you don't insist on shooting raw, the sony cams are nice too for UW. But no viewfinders. Most of them have M exposure mode. And ikelite makes housings for all of them.