Underwater flashes are important if you want to restore colors lost at depth.  The brain is capable of interpreting what colors might look like so we tend to be disappointed with our images taken solely with natural sunlight.  In order to create colorful images a flash is often necessary, either from the camera's built-in unit or a more powerful external unit commonly referred to as a 'strobe'.  Compact cameras make good use of the built-in flash but DSLR users will need to invest in one or two external strobes.

BACKSCATTER

Built-in units are handy for Point & Shoot users but have the distinction of generating lots of unsightly spots on images taken in all but the most ideal conditions.  This irritating artifact known as backscatter results from flash illuminated bits of sand, debris, plankton, or tiny air bubbles.  Backscatter is especially evident in front of a dark open-water background, while a complex background (sand) hides it.  Backscatter can only be reduced by either waiting for the water to clear and by positioning an external flash as far as possible from the camera, aimed at the subject with a 45 degree angle.

EXTERNAL FLASH

External flashes (strobes) are essential for those who want to expand their capability.  Strobes deliver much more power than built-in units, allowing the use of lower ISO settings for less digital noise and smaller apertures to increase depth-of-field.  They also require mounting hardware and cables or slave sensors to communicate with the camera.  Cost is considerable and care must be taken to make sure everything is compatible as many film-era strobes and cables don't work with digital.  Ikelite sells a variety of reliable equipment and their website is an excellent resource.

Point & Shoot cameras generally trigger an external strobe using a slave sensor pointed toward the built-in flash or fiber optic cable.  For cameras with a hotshoe, a hard-wired connection to the strobe is best, especially if the combination offers true TTL flash metering.

BEAM ANGLE

Macro and close-up work can be done with a small strobe but wider scenes will require a greater spread of light from a large wide-beam unit.  Large units will be 2 to 3 times as powerful with 10 to 30 degrees greater coverage, making aiming less critical.

MULTIPLE STROBES

Adding additional strobes produces a number of effects.  Dual strobes are often used to soften hard shadows created by a single unit or spread light across a wide area.  Varying the power output and and position can produce creative ratio lighting.  Remote slave flashes may be used to light complex scenes such as caves or wrecks.