Check that the flash synchronises with the optimum action of your camera shutter by trying different settings at home and noting which one works best.

Because there will be not much water between your flash and the subject when using your camera in macro mode, you can even experiment in the dry with varying exposure settings. Be aware that the amount of light is inversely proportionate to the square of the distance it covers. Get close.

Use a translucent diffuser over the flash when shooting wider shots. Although doing this reduces light output, it ensures the best coverage of the light and less critical aiming
of the flashgun.

Position your flashgun at the end of a long mounting arm attached to the camera housing to ensure that it’s a long way from the optical axis of the lens. This avoids revealing the detritus in the water, otherwise known as backscatter.

Otherwise, turn your flash away from the lens so that the cone of light immediately in front of it doesn’t light up any detritus within the camera’s view. This is often known as “edge” lighting, because you use the edge of the beam of light so formed.

Always enter the water for a dive with a fresh set of batteries installed in the flash. Rechargeable ni-mh-type are the most reliable.

Keep your flashgun scrupulously clean and check the lightly greased O-ring for foreign objects and correct alignment in its groove, just as you would your camera housing before sealing it up.

If you use an electrical synchro cord, treat it with care, because it’s delicate. Carry a spare.

Make sure the camera synchronises and the flash fires by trying a test shot just before you commit to the water.

Have the patience to wait for the flash to recycle after each shot. Even a couple of seconds can seem a long time during frenetic action.

Experiment with the camera’s “manual” mode so that you can understand the effect of changing the shutter speed (which affects the ambient light exposure but not the flash) and of varying the lens openings (apertures) and flash power-settings, thereby balancing the light of the flash with the ambient light.

The flash will usually be the dominant light source, so don’t white-balance the camera for the ambient daylight, and don’t use colour-correction filters in combination with flash.

Try positioning the flash in places other than directly alongside the camera. Try side-lighting and back-lighting. Experiment boldly. If you let the flashgun’s tube cool for several seconds between shots, you can even experiment at home in the dry.In this photo the flash is 1m from the subject

As water absorbs the light from a flashgun, it reduces the quality of the colour we see when the light reflects off the subject. So the more distant the subject, the further the light has to travel and the less colour we see when the light is reflected. In this photo the flash is 3m from the subject