Depending on what features you are looking for it can be very difficult to find at a decent price. I spent months looking for a camera with pan tilt and zoom that was underwater safe. Only options I found were intended for places like public aquariums, cameras starting in the $2500 range and going over $10k. I ended up putting a 10 gal tank just below the surface and putting a regular IP camera in that. Whole lot cheaper.
If you just want a straight on shot there are a few options out there, not sure how good they are though.