I (actually, the wife) has used S80 in the canon housing for a couple of trips.
Main reason to get it was small size.
Here is our (limited!) experience, using it mainly in 'P' or scene (UW) modes.
(a) AF is good topside, but not tack-sharp underwater, even in good viz.
(B) Colors are better than what I get from my old Olympus C5050.
© Using internal strobe is a pain -- close-up shots often get burnt,
and then it takes several button-presses to change flash power.
I suppose this will be less of an issue if you use external strobe.
To see what we have been able to do using these cameras, you can
check some photos from my website (S80 used only in the last
two trips):
Ajay's Webpage
If I have to choose again, I may look more seriously at Fuji E900, which
has been recommended here by more capable shooters..
Best,
Ajay.