visible light - Why is water blue (on a quantum level)

OK, lets formulate it differently and say water works as a blue passing / red restricting filter.

It is actually observable. Just do a dive in a swimming pool with white light (maybe even at night) and a pool with a metal casing or white tiles. The further you look, the more blue it is.

So why is this so? Why is water not transparent?

And how does it happen on a molecular, subatomar or electron shell level?

Do 'red' waves become absorbed and retransmitted as 'blue' light waves or do only the 'red' waves get absorbed and the 'blue' can pass?

Can Maxwell help?