For closeup or nighttime video, you turn on the lights and click the filter away. The goal is to have the color balance as close to what your eye sees as possible. And speaking of power, you need more than one battery. On many dive boats, there may not be enough power outlets for you to recharge your camcorder battery between dives. If you're shopping for both a camcorder and housing, talk to your dive store or factory rep about systems.
Some camcorders work better underwater.
Check the lights. If you're doing a night dive, check for the color-correction filter. Some of the most fabulous nighttime critters I've seen appear bright red in my video footage, because I forgot to click the filter off. Give your video housing a practice dive, preferably in the rinse tank, before you get in the water with it. Once you're in the water, stop at about 15 feet.