I have bought the Canon S100 as a second camera (a Nikon DSLR is my main camera) and I like the idea to protect it to depths of up to 40m with the optional WP-DC43 waterproof case.
Indeed, whereas you can find a lot of underwater point-and-shoot cameras, none so far can shoot raw, and their sensors, even for the best (Panasonic DMC-TS4), are no match to the S100. Conversely, the cameras with better sensor usually propose underwater case at a price far higher, and are usually much bigger and heavier.
So the S100 + WP-DC43 looked to me like a nice compromise: not cheap but not too expensive for a full manual-raw camera with a great (small) sensor, a small and light body, and very capable of taking great pictures.
Whereas I have not yet dived with both, I have made my first tests, and I have been struggling finding online reviews from users. So these are my first impressions, which are, so far, the only genuine ones as far as I know:
I like: many details are making it “almost” professional: light diffuser for the flash (works quite well if you are not in macro mode and remember that the S100 flash is not exactly the best for its class), you can use a tripod. It’s easy to put the S100 inside the case and to remove it. The leash works quite well too. Last and of course not least: easy to handle, controls and buttons can be used with gloves, it’s really easy to use it in the case. It is even somewhere easier than without!
Issues: you cannot use any longer the rear wheel! You can still use the front one however. So basically, the S100 is not as seamless to use manually as without the case, but you can still have full control. It is an issue you can live with, but that’s a real one. Further more, there is no cap for the front window of the case, that’s really too bad. And there is no bag either for the case itself. One made in neoprene would have made sense. The light diffuser’s leash cannot be attached to the body without some additional small carabiner (not included of course). The buoyancy of the case and the camera altogether is really positive, you really need to add some weight to make it neutral. Of course it is written nowhere in the S100 mini-website (or show me where please), but you can purchase the weights (Canon WWDC1, image on the left), actually you really need them for diving.
Image quality for underwater pictures: remember, the S100 can shoot RAW, has some very nice low light capabilities and can open at f/2.0 at 24 mm, but cannot be synchronized with additional flash but the small one embedded. Please also note than the sensor is still quite small even if bigger than most of the point and shoot. Therefore, the depth of field is really significant, often too much. Conversely, in macro mode, that’s making the shooting straightforward compare with APS-C or FX sensors.
An extreme shot, close to some dangerous wildlife, WB auto, no flash, low light.
Conclusion: the S100 and its underwater case are not cheap, but they are a very decent option to take serious underwater pictures, for the fraction of the price of a DSLR and its 40 meters depth underwater case. However, they are issues and limitations, which could not be negligible for some, but which I believe to acceptable, even if regrettable for most of them.