This got talked about a month or so ago, so maybe do a search on the S5.
I'm not a fan of the S-series for underwater. They aren't great on macro from my experiences, which seems to be standard for most big zoom P&S's. There's no way you'd ever want the 12x zoom underwater either. Remember the golden rule, get close to your subject!
The other thing, the Ike housing for the S5 is a LOT bigger than the Ike G9 housing.
If you're wanting more zoom on the G9 for above water shots, you can add a teleconvertor to the camera.
If you're tossing up between the 2 for underwater, IMHO, the G9 is by far superior.
Ryan.