eli5 Why is water transparent in a glass but blue when it is in the ocean?

Question

My wife gave birth to a philosopher. If you come up with an experiment I can show my 6-year-old son I’d appreciate it.

In: Earth Science

Answer 1

My understanding is that water in the ocean is blue/green because its reflecting the sky on a background of darkness. Keep in mind that the deeper you go in the ocean, the less light there is, so eventually you reach pitch black. So the surface layers of the water are effectively lying over that black level, and that forms a sort of mirror (very sort of) which results in a sort of reflection of sort of the colour of the sky.