Another question we get from our readers a lot at Popular Photography is, "How can I shoot great underwater photos?" There's a couple different ways you can do that.
You can go out and buy an underwater camera. Pentax makes a whole line of rugged cameras that do really, really well under water, that I highly recommend.
Or you can get an underwater housing for your camera you already own. Most manufacturers out there make their own proprietary housings, and there are also third-party housings out there, as well.
It's recommended, if you are looking into getting an underwater housing to shoot in the ocean or the pool, don't skimp out on the housing, because obviously the cheaper brands are more susceptible to leaks, and they might give you less tactile control of your camera.
Again though, it's sometimes better to go for a camera specifically made for shooting under water, like Pentax's Optio series, because they have things like LED lights around the lens, which will illuminate your subjects under water and just give a really nice photo; whereas if I had this in a housing, I wouldn't have that advantage.
Another thing to remember is as you swim deeper, so let's say you're scuba diving with your camera, the deeper you go, obviously the more light you're going to need; whereas if you are shooting just your family in the pool, on vacation or something, you're not going to really need to worry about that so much.
Again though, I can't stress it enough: Do not buy a cheap underwater housing. You will regret it in the end.