The different wavelengths of light are absorbed at different rates in different materials. Ocean water absorbs red fastest, then green, and blue slowest. In deep ocean water the red and green are absorbed while the blue is scattered and reflected back. Nearer to shore, with less depth, the green is also reflected, resulting in a blue-green color, while nearer still, the color of plants, sand and sediments in the water add to the mix, making it greener still.
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