Check the waterproof housing for your camera by dunking it into a tub - without the camera inside, of course - before you test it on a proper watery adventure, recommends the Imaging Association of Germany. Make sure you pay special attention to the rubber seals during the test.
But, just because the housing survives the tub test, it doesn't mean you can relax when you're out in the field. If you're underwater and notice air bubbles coming out of the housing, there might be a problem. Get the camera out of the water quickly, since it's likely the housing is leaking.
Clean the housing after every dive, especially forays into salt water, with a lint-free cloth. Never use cotton swabs, since they leave lint and the smallest particle can lead to leaks.