Why is the ocean blue?

Why is the ocean blue? I know there is a scientific answer. I knew the answer as a child but have forgotten years ago. Is it something to do with the the sun? Rays from the sun? Or the sky? This question popped into my head and I can't forget it.

Water has a very slight blue colour that can only be seen in large volumes. While the sky's reflection does contribute to the blue appearance of the surface, it is not the primary cause. The primary cause is the absorption by the water molecules' nuclei of red photons from the incoming light, the only known example of color in nature resulting from nuclear, rather than electronic, dynamics