The proper use of flash or strobe lighting is one of the most difficult yet fundamental skills of underwater photography. Learning to compensate for the physical differences between water and air is vital. Water refracts light at a higher index than air, magnifying objects up to 25% closer than they appear. Water also drains color at increasing depth and distance, particularly the longest wavelengths of color, or 'hot' colors such as red and orange. Photos taken without underwater lighting look drab and blue, due to this wavelength absorption. A strobe or flash supplies light at high speed and at varying levels for optimum color and clarity depending on the subject and setting.

The camera flash and/or strobes are used to supplement the overall exposure and to restore lost color. Generally they should not be used as the primary light source. Ideal image quality is achieved when the ambient, or natural, light from the sun is clear and undiluted by floating particulates or strong ocean currents. In situations such as the interior of shipwrecks or sea caves, wide-angle images can be shot exclusively with strobe lights, but this is the exception to the rule. Photographers strive for an aesthetic balance between the available ambient sunlight and artificial strobe lights. Strobe lighting by itself without diffusing panels can create a harsh spotlight that has a sharp contrast. Deep, dark or low visibility environments can make this balance more difficult and are generally avoided, except when the goal is to make blurred or dark shots to evoke mystery.

When using strobe lighting the photographer must be prepared to position the strobes in such a way as to prevent backscatter.
Backscatter, which appears as hazy dots in an underwater photo, is common and mainly caused by strobes or the camera flash lighting up particles like sand or plankton in the water between the lens and the subject. It can also be caused by lighting up the water directly behind the subject, and is easier to detect against a dark background. The best technique to avoid backscatter is to position two strobes far from the main body of the camera lens, lighting up the subject with the edge of the light beams. Various systems of jointed arms and attachments may be used to manipulate off camera strobes.

There may be situations in which the photographer sacrifices the aforementioned precautions against backscatter. For instance, when using a macro lens for very close range photography there is a loss of light due to the lens being moved further away from the camera sensor. Because of this, the viewfinder will be slightly dimmer. This issue is resolved by moving the strobes closer to the body of the camera and aiming them at full strength ahead. The subject is normally very close to the lens, and available sunlight often fails to provide adequate light for sharp, clear photos.

Ambient sunlight may be used exclusively for a photo shoot, but is not recommended. In shallow water, using custom white-balance camera options may provide excellent color without the use of strobe. Using color or gel filters over the lens to overcome the blue-green shift may balance color settings. The quality of color would vary with depth and water visibility, and there would still be a significant loss of contrast without strobe or flash lighting. Using color balance options on digital cameras often 'warms' or 'cools' the entire frame, often ruining the shot, and should be used as a last resort. Often this is a hassle to correct in postproduction and it is always best to create the optimum lighting at the time the image is captured. Ambient light photography underwater is ideally used with silhouettes, light beams, and large subjects such as whales and dolphins.

Digital cameras have drastically improved many aspects of underwater imaging, but it is unlikely that the use of flash lighting will ever be eliminated completely. From a practical and aesthetic standpoint, the use of flash remains essential in maintaining image clarity, color and sharpness in an underwater environment. The loss of color and contrast is always best addressed by preparing a photo shoot with adequate lighting, both ambient and artificial.

Efforts may be taken to correct lighting mistakes in postproduction with software such as Adobe Photoshop, or lesser programs such as Picasa and iPhoto. Yet it cannot be overstated that the most balanced, beautiful photograph is best when post editing is minimal and the preparatory lighting setup has been thorough.