A dive light will not increase the Viz from 10ft to say 12ft but what it will do is help your buddy see where you are. The only time when a torch will help is when you are looking under some thing. Narrow beam torches are the best because they seem to cut through the water like a laser beam. The more powerful the better. I like HID torches due to the pure white beam, but HID torches are not good for underwater Videoing.