Why is the ocean blue?

The ocean appears blue primarily because of how water absorbs and scatters sunlight.

When sunlight enters the water, it is made up of all the colors of the rainbow. Water molecules are very good at absorbing the longer wavelengths of light—the reds, oranges, and yellows. The shorter, blue wavelengths of light, however, are not absorbed as easily. Instead, they bounce off the water molecules and scatter in all directions, which is the light that eventually reaches our eyes.

While the ocean can sometimes look green, grey, or brown, that is usually due to factors like suspended sediment, organic matter, or tiny organisms like algae reflecting those colors back to us. In deep, clear water, the blue color is most prominent.