Water is clear. The sea appears blue due to light refraction/reflection.
Some science:
As we live on a pebble mainly covered with water, and as this pebble bears the pretty name of “the blue planet”, it seems legitimate to wonder about the reasons of a such uniformity. All the more because the blue mirage that can be seen from space is less visible when our feet are in the water.
We have all noticed that if the sky is blue, the sea dons its blue clothes, and if the sky is gray, the sea’s dressed in dark. Would it mean the sea is endowed with the ability to change its looks according to its mood? That’s not likely. On the other hand, a more coherent explanation, at least for its surface, is that the colour of the surrounding sky reflects itself on the sea and gives it its colour.
As to the deeper layers, the blue colour is given by the penetrating sunbeams. When a sunbeam hits the water surface, it doesn’ stop its course at once, but splits into different hues as it goes deeper. The colour yellow disappears along the first meters, then the red fades, and finally the green. The only colour left is blue.
http://pagesperso-orange.fr/dit_papa/us/dprep03u.htm
