I have used the Canon G9 & G12/canon housing, Olympus E-PM1/Olympus housing, Olympus E-PL3/Olympus housing and Olympus E-M5 and Nauticam housing.
I like the Olympus cameras better than the Canon - to me they focus much faster. The Olympus housing is very durable but very difficult to change ports and not as ergonomic as the Nauticam. I am using the Olympus E-M5/Nauticam housing love it - the housing is smaller, better ergonomics and port changes are so easy.
Here in the states there is a special on the E-PM1 & Olympus housing for $500 USD. This is a great starting system - you can always add lens/ports. The nice thing is if you decide to upgrade (down the road) to a newer Olympus micro 4/3s camera the lens will still fit any of the micro 4/3s. I have several lens from my E-PM1 that I now use with my E-M5.
I shoot in manual mode taking RAW photos which all of the cameras above accommodate.