From a budget standpoint, the S100 and housing seems like a great option (and one I'm considering, myself). Personally, I don't see much about the S110 that makes it a step up from the S100.


As for the housing, the Ikelite one is probably better quality for not too much more money (B&H sells it as a kit with the S100 for $600). The Canon one would likely be just fine for above water and snorkeling, but for diving I'm inclined to the Ikelite.


The other option I'm considering is the Sony RX100 with the Ikelite housing. That big (relatiely) sensor would be good for the higher ISOs needed underwater (although for diving, a strobe will still be needed).