The 2 things you're looking for are slightly incompatible - the reason being that the best compact cameras such as the S100 that another reader recommended are not waterproof, while all the waterproof cameras are fairly poor in low light....
You might be better to get a cheap(ish) camera for underwater shots, or even a waterproof camcorder which does stills such as the sanyo CA9 - and compliment that with a good compact camera such as the S100 or Olympus XZ-1 which will cover the family photos and work great in low light situations.
Hope it helps!
