The ocean is blue because it
reflects the
sky's blue
colour.
To see why the sky is blue click here.
You will also notice that on a cloudy day when the sky is grey, the ocean is grey too (it reflects the sky).
When the ocean looks blue-green it is owing to high concentrations of plankton.
why-is-the-sky-blue.org
i can't believe this node didn't already exist.