Water selectively filters out colors of light passing through it. Red is mostly absorbed at ten feet. Orange and yellow wavelengths are gone by 25 feet, leaving a blue cast. At much greater depths only violet and ultraviolet light remain.
This effect occurs not only to light traveling downward into a column of water, but also to light traveling horizontally underwater.
A bright red shirt seen 50 feet away through clear, shallow water will appear just as dark and colorless as the same shirt seen up close at 50 feet of depth (above).
Photographers use a flash to restore colors in deep water. The ability of the flash to enhance warm colors also diminishes rapidly with distance. The painting above, from Dinotopia: The World Beneath, shows Devonian sea creatures is if lit by an underwater flash.
The submersible and marine reptiles in this painting are rendered in grays, blues, and greens. All the reds from the brass parts are missing, so those areas appear more greenish. The far sea creatures have lost almost all contrast, and nearly match the background color.
Various impurities discolor water in different ways. Silt or clay gives water a brownish color and visibility drops dramatically. Algae growth, typical of freshwater lakes, gives water a greenish appearance.
Photo from Deepsix.com.