Generally speaking, the act of white balancing is meant to calibrate your video camera so that it captures color accurately. The theory is that if you can tell the camera which color it sees is actually white, then the electronics in the camera will adjust everything else in the scene to the proper color. We've all seen video that's not been properly white balanced. For instance, you're shooting your kids birthday party in the living room and follow the kids outside as they go to play with their new toys. Everything looks good when you're inside, but as you walk outside with the camera everything outside looks blue. This has to do with color temperature. Basically, sunlight has a high color temperature and incandescent (indoor) light has a low color temperature. To the eye, sunlight looks "cool" or "blue", while indoor light appears "warm" or "orange/red". In this case, the camera was set for indoor white balance, making the outdoor color too blue.
When shooting video underwater, the diver faces even tougher issues. As we all learned in our first scuba classes, water absorbs light. As we descend, more and more of the red end of the spectrum is absorbed by the water. This can really play tricks on your video camera, often resulting in extraordinarily blue footage. There are a number of techniques to help compensate for this color loss; using lights, using filters and white balancing are chief among them. I'm going to stick with white balance here ... lights and filters for another day.
To white balance your camera, if you have a camera and housing that gives you the option to do it manually, you will want to point your camera at a white object (preferably a clean white slate - like this one from Gates), zoom in so the white fills the majority of the screen, and activate the white balance control. If your video camera only has auto white balance, you can often set it by zooming in to the white object, turning the camera off, then turning it back on while still focused on the slate. It should read the white slate as white when it turns on. Other cameras with auto white balance will adjust if you hold steady for a few seconds on the white card. I strongly suggest looking for a camera/housing combination that allows for manual white balance control.
A few things to keep in mind:
1 - If you are going to be shooting at 100 feet, don't white balance at 15 feet and then go shoot. Remember, as you descend there is less and less red light available. Generally, I try to white balance with every ten feet of change in depth.
2 - White balance with your red filter in place. This usually results in the best image, although conditions can vary so much it's worth experimenting.
3 - What if you don't have a white slate available? There are usually a couple of options on any dive. You can zoom in to the sand and white balance. You can use the sun as white. You could even laminate a plain white sheet of paper, have your buddy stick it in her BCD pocket and use that.
As you continue to dive and shoot more underwater video you will need to experiment with different methods of white balancing that work best for your particular camera. Remember, this is part of the fun. Get out there and dive and enjoy your video camera!
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