Using a strobe light

There are some does and donts to using a strobe. While the strobe will produce a powerful burst of light, its energy will still be absorbed by water. So get as close as you can to your subject, when you think you cant possibly get any closer, get closer still. This will reduce the amount of water between you and the subject, and so allow as much light as possible to reach it from your strobe. You should also place the strobe light on an arm, so the light comes in from a different angle, this will reduce the amount of back scatter, caused by light hitting particulate in the water between you and your subject.

Also, have a thought for the sensitivity of the creature you are blinding with flashes. The majority of marine animals dont have eyelids, or any way of protecting their eyes from bright light. A half hour photo session in the midst of a group of over zealous divers probably wont do their nerves any favours.