Why is the ocean blue?
The ocean appears to be blue because the shorter blue wavelengths are reflected off the surface of the water.
The light that is observed is the light that is reflected. The longer wavelengths of light penetrated deeper into the water and are absorbed. The shorter blue wavelengths are reflected off the surface of the water. On a grey day the ocean will appear to be grey because that is the only light that the ocean can reflect.
Water that is more shallow will also reflect green light appear a teal color. Water that is very clear may also reflect the yellow color from the sand beneath also changing the color of the ocean.