When light enters water a number of changes occur. The intensity is reduced, the quality is altered and it is refracted. On the surface the visible light is made up of all the colours of the rainbow, but under water the different wavelengths of colour penetrate to different depths. Red for example is absorbed in shallow water and blue penetrates the deepest. Because of the differential absorbance of light at depths greater than 25 feet most available light, most available light photographs are rather monochromatic. At 15 feet red is not distinguishable in available light, at 30 feet orange disappears and below 60 feet yellow is invisible. To show the true colour of subjects supplementary light is needed as either fill or keylighting. The use of artificial light underwater adds colour contrast to the image, depending on light source and distance may increase depth of field, and having control over the placement and direction of the light lets you create images that were not possible with the existing available light.
The cameras focal plane shutter can be synchronized with flash at 1/30, 1/60 or 1/90 sec.