my wife bought me a german-made enclosure for my digital camera for christmas.
basically it is incredibly difficult to shoot, usable shots are on the order of 10:1 or 20:1 even after I had a lot of practice. so i'd strongly advise against a disposable $20 dollar underwater camera with only 14 shots, or any film camera for that matter.
the fish are quick and darting and you can't put the camera to your eye to look due to the swimming motion, you have line it up with your line of sight extend your arms straight and hope for the best, just like you see in the documentaries on PBS.
If you have a powerful flash, it is probably better to use it, since the color distortion underwater really varies depending on condition. however, it might damage sensitive wildlife like squirrelfish etc so i'd use your discretion.