Not that I am really knowledgeable to give advice, but I have tried several compact solutions underwater, including Canons, Nikon and Panasonic.
If you want to a good compact, I'd definitely go for Canon G10 for several reasons: has 28mm. lens, shoots RAW without needing to "patch" the sw, has a hotshoe, does REALLY GREAT macro focusing at only 1cm. The other option is the Nikon P6000, and it's a small step back form the G10 in terms of specifications.
However, you won't be able to use most of those advantages if you get the original Canon housing.
Ikelite and another companies like Patima and Seatool offer a very functional housings for the G10, but they the last two are well demanding on your pocket. The Ikelite housing #6146.10
has the advantage of the integrated TTL circuitry and a bulkhead. This allows you to use wired Ike external flash in full TTL mode, whenever you want to. You can always use the integrated G10 flash, but that's not a good idea in visibility less than perfect due to backscatter issues. Besides the port of any housing for that camera blocks the light of the internal flash! This means you get a hard shadow in the lower right portion of every frame.
Ike also provides a domeport for this housing allowing you to use the full advantage of the 28mm. lens.
The french company Dyron (
Dyron ) recently released a couple of replacement ports (short and long) designed especially for the Ike G10 housing. They allow you to use the Dyron domes and macro lenses.
This means that your camera will have the greatest support in terms of independent accessory providers.
If you use the original Canon housing - you're more or less stuck with what you get. You'll be able to use only optically triggered flashes and the selection of accessory lenses is extremely limited, unless you engage yourself in a quite complex DIY project for altering the housing.
As for the Oly compacts... Hm... I never used those underwater for quite many reasons.