Why is water in the ocean blue?

This is all to do with how water interacts with light. When water is a vapour in the atmosphere, the molecules are not glued together in the same way they are in a liquid. Water molecules tend to stick together with hydrogen bonds, but in a vapour they can spread out. In this state they are very good at soaking up infra-red light, but not visible light.When water forms a liquid - lots of molecules stuck together - it stiffens the bond between the hydrogen and the oxygen, and instead of absorbing infra-red, it now absorbs light at the red end of the spectrum. White light is a mixture of lots of different wavelengths, and if you take out some of the red light, it looks a little bluer. The more water you have, the more red is absorbed and the bluer things look.