When shooting underwater with artificial lights, there are only a few key things you need to know. The first is how to avoid backscatter. Backscatter, besides being the coolest store on the planet, is any particulate matter that reflects strobe or video light back at your lens. Backscatter is the stuff that makes your footage appear as though you are shooting in a blizzard of plankton. It will hide your intended subject behind a cloud of white glowing dots and very likely ruin your shot. This phenomenon is exceptionally severe in flash photography. The flash will freeze particles, in all of their glowing splendor, directly in front of the camera lens.
Essentially, you want to keep the water between you and your subject as free of light as possible. To accomplish this, you must understand that the beam of light projecting from your light head extends at an angle. HID lights are the widest at 80 degrees to 110 degrees, while most Tungsten lights are around 60 degrees to 90 degrees. The key is to use the angle of light to your advantage. Don't point the lights dead center on your subject. Angle them so only the light beam's outer edge touches your subject. This creates a cone of dark water in front of the lens that extends to where the two light beams meet your subject. The farther your subject is from you, the farther you should move the lights away from the camera housing. This increases the length and breadth of the cone of darkness between the lens and your subject. If your subject is closer to the camera lens, bring the lights in toward the housing to reduce the size of the cone of darkness. When done correctly, only your subject will be lit.
Above: This is the proper straight-on positioning for underwater lights. Notice how you use the light beam angle and not the direct pointing of the lights to expose our subject. The cone of darkness in front of the lens then keeps the backscatter to a minimum.
Above: This is the improper position for video lights. If you point the lights directly at your subjuct you will expose the particles in front of your lens and increase backscatter.
For macro shooting, a slightly different technique is required. Because you are closer to the subject, less water exists between the lens and the critter. This reduces the density of particulate that causes backscatter. The intensity and positioning of the lights is now your main concern. Your shadows will be sharper and more distinct, and your highlights can become overexposed. To reduce harsh shadows it is best to position one light at a 45 degree angle to the face of your subject. This is the key or main light and it illuminates the primary aspect of your subject. Usually the subjects face. The second light is used as a fill light. The fill's primary function is to reduce shadows cast by your key light. I usually place the fill above my lens and point it directly on my subject.
Above:This is the proper lighting position for close macro work. The key light exposes the face of your critter and the fill fills in the shadows cast by the key light.
Keep an eye on your exposure by using the zebra stripe setting of your camera. Zebra stripes tell you where highlights are at absolute white. If there are too many solid white highlights, your footage will look blown out or overexposed. For those who cannot manually control your camera's exposure there is not much you can do except move back from your subject or adjust the power settings on your light system. If you have a camera that allows you to manually control your exposure you can dial your iris or shutter speed down for a more even exposure. For more information on how to manually control your exposure please refer to my article entitled Manual Controls in Video Cameras
The next thing you must be aware of is color and white balancing. For those of you with housings that do not allow manual white balancing (w.b.), you can achieve great color with auto w.b. For tungsten lights, place your camera on auto indoor w.b. This will w.b. your camera to the warm light cast by your halogen lights and in doing so make your background appear bluer. If you have HID lights, daylight auto w.b. works, but only if you're using the lights as a distant fill at or around 3-4 feet from your subject. This is because HID lights are daylight balanced. Any closer than 3 feet to your subject and HID lights will appear warm and indoor w.b. works best. Never use a red filter when using auto w.b. modes as your camera will not accurately compensate for the color shift and all of your footage will have a reddish cast.
The best way to adjust white balance when shooting with lights is to do so manually with a housing and camera that allows access to this feature. Point your camera toward a white object, (I use a white card, my hand or light colored sand) and manually white balance it with the lights on. As you change depth and the strength of the ambient light varies, you will need to white balance again.
Good luck and good shooting!