>>2973948is right, going deeper than ~20ft will make everything look blue, but that can be solved in post by adjusting the white balance, so get a camera that can shoot in raw. There's no "waterproof" camera like the TG-3/4 that will be good down to depths you'll actually go diving, so plan on buying an underwater case. Find a used S100 that'll shoot raw and be compact in the case.
This is a good investment because when you're ready to upgrade your underwater photos you'll get much more benefit from getting better lighting rigs than upgrading cameras.
