The Otago University Tramping Club, asks :-

Why is water so blue in the deep bits?

Richard Dowden, a physicist recently retired from Otago University, responded.

White light is made up of the colours of the rainbow: red, orange, yellow, green, blue, indigo, violet (ROYGBIV).

Water is almost opaque at the far red end (and out into the near infra-red), but almost perfectly transparent at the blue end. In fact, in the old days of movie projection using arc lights and (highly flammable) nitrate films, the light was passed through about 50 mm of water to remove the infra-red radiation and so avoid igniting the film if the projector motor stopped. This still left all of the visible light (ROYGBIV) so the movie screen would still appear in black and white (they didn't have colour movies in those days).

Looking at white light through increasing thicknesses of clean water, or looking vertically down at white sand at the bottom of increasing depths of clean water, the colour you see would be made up of ROYGBIV for very shallow water and so will still appear white, OYGBIV = "hard" white for water a few metres deep, YGBIV = green for deeper, GBIV = ultra marine for deeper still, and BIV (or even IV) = deep blue to black for very deep clean water.

Salts dissolved in water, as in sea water, generally enhance this effect. In fact, a 2.5 per cent solution of cupric ions is most effective in removing IR (and R,O,Y,G to a decreasing extent) and appears blue in thicknesses of only 20 mm.

Looking at a lake or the sea at low angles the colour is governed more by the reflection of the light scattered from the sky: blue on a clear day, grey on an overcast one.

Why is the sky blue? That's another question and has an entirely different answer (in short: blue light from the sun is scattered at large angles by the air molecules far more easily than red light is and so doesn't appear to be coming from the sun's direction at all).