An introduction to light in an absorbing, scattering medium is offered, with application to correcting the colors in underwater photographs. The "waterlight" model is presented which quantifies the amount of light scattered in the direction of the camera from the medium itself. A spectral model for ocean water is described along with a method to represent it in three bands (e.g. RGB). Given these models, the radiance of a diffuse reflector at known depth and distance can be computed. At infinite distance, this becomes the "abyss color", a new and useful concept with which to estimate camera response, and the water parameters. The color correction procedure for a given depth and distance is outlined and illustrated. Unknown causes of color shifts from camera or water are addressed via a "blue balance" transform, which maps the recorded abyss color to the modeled abyss color. A compact vector expression for the correction is presented and examples where it is applied to underwater scenes are provided.