White light consists of all the primary and secondary colors in the spectrum. Each color is distinguished by the degree to which it scatters and absorbs light. When sunlight hits seawater, part of it is absorbed while the rest is scattered in all directions after colliding with water molecules.

When sunlight hits clear water, red and infrared light absorb rapidly, and blue the least easily. According to Curtiss O. Davis . . . ‘only blue-green light can be transmitted into, scattered and then transmitted back out of the water without being absorbed.’ By the time the light has reached ten fathoms deep, most of the red has been absorbed.

Why doesn’t tap water appear blue? Curtiss continues: ‘To see this blue effect, the water must be on the order of ten feet deep or deeper. In a glass there is not enough water to absorb much light, not even the red; consequently, the water appears clear.’

Thus if clear water is of a depth of more than ten feet, it is likely to appear blue in the sunlight. How can we explain green and red oceans’

Both are the result not of the optical qualities of sunlight but of the presence of assorted gook in the water itself. A green sea is the combination of the natural blue color with yellow substances in the ocean—humic acids, suspended debris, and living organisms. Red water (usually in coastal areas) is created by an abundance of algae or plankton near the surface of the water. In open waters, comparatively free from debris and the environmental effects of humans, the ocean usually appears to be blue.

—David Feldman, from Why Do Clocks Run Clockwise? and other Imponderables, 1988.