S95 or S100 for shooting under water?
S95 is cheaper, which is good in the unlikely case that the underwater housing fails. However, the S100 has 24mm equiv. wide angle where the S100 has 28. Field of view is narrower underwater due to the water's index of refraction. Also, it is wise to get close up to what you want to photograph in order to maximize visibility (light fall-of and debris will mar pictures of more distant objects). These two facts together mean that wide angle is where you want to be - the wider, the better.
Also, the S100 has 1080p video where the s95 has 720p. This can be an important factor (underwater videos can be very striking).