As for S90 vs G11. They have same sensor, same wide angle, which is more important for UW. Image quality will be very close. S90 has faster lens at wide end, f/2 vs f/2.8 of G11, so at low light without flash, S90 will produce better picture at wide end.
What G11 wins is handling above water, more button, so less likely need to go int menu. Longer range on long end. Bigger battery, so better battery life. More powerful onboard flask, Also hot shoes for flash.
Now the big question here is for under water usage. I will highly recommend S90 over G11. Reason is with Canon case, they camera become a lot bigger. S90 isn't small anymore, G11 is really big with case on. make it harder to handle while dont produce better pictures.