You might think that absolutely pure water would be perfectly clear
and utterly transparent, but it's actually blue. The blue color of
the water in the oceans (and not the blue of the sky) is the reason
why Earth is mostly blue as seen from space.
Pure water absorbs some of the light that passes through it. It
absorbs red light more than yellow, yellow more than green, and
green more than blue. Only the deepest blue light can travel very
far through water, so a large mass of water takes on a deep blue
color.
The blueness of water is easily visible in a swimming pool lined with
white concrete. It's even visible in a white porcelain bathtub. But
the bluest water of all is the clear tropical ocean far from land,
where the sea is much bluer than the sky.
The color of the ocean is strongly affected by plankton and impurities
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