This discussion is not aimed at underwater photographers, rather all types of above-water photography, specifically Aerial Photography.
Contrary to all desires of adding light underwater, above water photographers look at removing or limiting light to achieve special effects. This can be done by using Neutral Density Filters or Polarising Filters. I'll also touch on use of the ever-popular UV Filter;
UV Filters are widely used as nothing more than a clear piece of glass to protect the camera's lens from bumps, knocks and debris. Many photographers believe (as was their original purpose) that these filters remove excessive blue tint from your images at high altitudes or when photographing blue water.
Polarising Filters work the same ways as a pair of sunglasses work with our eyes - they eliminate glare and add contrast. This is particularly important when photographing reflective outdoor surfaces such as water, snow, concrete etc... Talking tech - polarising filters will reduce the light by 0.3 of a stop. When shooting outside with the sun, this is not a huge change to how you take the photo.
Neutral Density Filters are tricky to master, but very important to film makers and photographers. When capturing video, the best results are achieved by reducing/increasing the camera's shutter speed to shoot at twice the camera's recording framerate. This ensures that each frame 'blends' with the next, offering a natural visual experience. When recording at 25 frames per second (fps), our aim is to get the camera use a shutter speed of 1/50 of a second. If you're shooting outdoors, a Neutral Density Filter can be used to 'darken' your image and achieve this.
In Photography, Neutral Density Filters are used for long exposures. Ever wondered how photographers manage to make lakes look perfectly flat even on a windy day? They us a Neutral Density Filter to force the camera to use a slower shutter speed (usually the shutter is open for several seconds) and 'blur' the surface enough to make it look peaceful and flat. This is also common practice with Waterfalls, Camp Fires and during night time with moving lights.