Couple of things worth pointing out out with that cheap camera.
It says 18megapixel, but that is interpolated, what is the native resolution of the camera's sensor? Second it can only record VGA (640x480) movies, which was fine, five years ago, but is woeful when even low end devices can achieve 720P these days.
Second the lens looks tiny, so low light performance is likely to be rank unless the device has a very good sensor chip in it (e.g. iphone type sensors- a very impressive device).
The Gopro camera Giovanni linked to is an excellent bit of kit, the latest version 3 model is breathtaking. They are a bit more expensive, and unfortunately the shape of the camera is rather boxy for submarine use- okay if you're clipping it to something large, but otherwise it will present a problem with drag.
If you want a good quality camera ca[pable of still and film, but don't want to clip it onto a sub, I would seriously look at the Panasonic FT20 or FT3 if you're feeling flush.http://www.panasonic.co.uk/html/en_GB/P ... index.html