Water is approximately 800 times denser than air which forces light to travel at a much slower speed. It also scatters the light as it loses color and intensity.

Now we have to consider the color qualities of light. Direct light from the sun is called white light. Though it appears to have no color, it is actually composed of all the colors in the spectrum. If you were to project it through a prism, you would see it break up into bands of color. Each color is a wavelength of light. Most people will remember this experiment from chemistry class at school.

When we descend below the surface, water absorbs the wavelengths of light selectively, one by one as depth increases. Though exact absorption rates will vary depending upon water conditions, you can assume that red will disappear at a depth of around 15 to 20 feet, orange at between 25 to 30 feet, yellow at 45 to 60 feet, green at 70 feet and at 100 feet, everything will appear blue or grayish green. At extreme depths all the light will be absorbed and everything will appear deep blue or black.