Why is water blue?
The water may be reflecting the color of the blue sky (or the reds of sunset, or the gray of clouds), but an intrinsic blue color is caused by the refraction and scattering of visible light by the water. Different water qualities and depth can change the particular hue of blue. Very shallow water, as in streams, appears transparent.
Visible light is composed of all colors and the ocean water breaks the light into its different components (think of a prism). The colors on the red end of the spectrum (longer wavelength) are absorbed and the blue reflected. A similar principle (called the Rayleigh Effect) makes the sky appear blue in daylight.