Why is the sea blue?
It's not just sky reflection — water itself is a blue filter.
1Many people think the sea simply mirrors the sky. In fact water molecules are picky about colour: longer wavelengths — reds, oranges, yellows — are absorbed more strongly, while blue is absorbed least and scattered most, so the deeper the water, the bluer it looks.
2The simplest proof: a bucket of water is clear, a deep pool starts to look blue, and the open ocean is blue through and through. Other things change the colour too — plankton-rich seas look green, muddy river mouths look yellow-brown, which is why not every sea is the same blue.
3Sky reflection adds to the effect, but even under an overcast sky, deep water stays blue.
Water absorbs red light more than blue — the sea makes its own blue.