The s100 is not a pricy camera and it will be foolish to spend a small fortune on a housing. In a year or two that pricy housing will not be worth much. In a market where camera manufactures update their offerings annually equipment does not hold value. The Canon housing is adequate and allows access to most functions. It costs a bit over $200 and my cannon housings have worked well. I generally shoot UW in M mode and RAW so that provides flexibility.
The question I would ask is what do you get for the extra $1000? If the answer is a $xxxx poorer well.....
Spend your money on a good flash. A good flash will run more than the camera. The good news is the flash will hold value and likely outlast the camera.
I shot my Avatar with a Cannon PnS in 2005. I purchased a DSLR housing shortly after but that's a different story.
The choice of the housing depends on the choice of lens. Without a doubt having a fisheye lens is a must to take great pictures so you need an housing that can have one.
If you look at the canon housing plus an Inon UWLH-100 28LD with dome you are talking about $1365 plus taxes
A recsea plus AD mount and Inon UFL165AD costs you $1460
So for $95 more you get an aluminum housing with access to all controls with a wider field of view and you don't need to buy or build a strobe mask as the housing comes with it bringing further savings, also you get a wider field of view
The only issue is cash flow as the entry point of the recsea combination is $799 whilst the canon starts at $399 but if you get all the way where you need to be, and no doubt a fisheye is where any photographer should want to be the difference is small to none and totally justified