The Book of General Ignorance: What Colour is Water?

"The usual answer is that it isn't any colour; it's "clear" or "transparant" and the sea only appears blue because of the reflection of the sky.

Wrong.   Water really is blue.   It's an incredibly faint shade, but it is blue.   You can see this in nature when you look into a deep hole in the snow, or through the thick ice of a frozen waterfall.   If you took a very large, very deep white pool, filled it with water and looked straight down through it, the water would be blue.  

This faint blue tinge does not explain why water sometimes takes on a strikingly blue appearance when we look at it rather than through it.   Reflected colour from the sky obviously plays an important part.  The sea doesn't look particularly blue on an overcast day.  

But not all the light we see is reflected from the surface of the water; some of it is coming from under the surface.   The more impure the water, the more colour it will reflect.

In large bodies of water like seas and lakes the water will usually contain a high concentration of microscopic plants and algae.   Rivers and ponds will have a high concentration of soil and other solids in suspension.  

All these particles reflect and scatter the light as it returns to the surface, creating huge variation in the colours we see.   It explains why you sometimes see a brilliant green Mediterranean sea under a bright blue sky."