The reason it’s blue?
The ‘deep blue sea’—and our ‘blue planet’ with it!—owe their iconic color to the light of the sun. When the sun shines on the ocean, the water absorbs the longer red and orange wavelengths of light while reflecting blue light back. This will only happen, though, with a large amount of water; the more water you have, the bluer it is.
This is why the water you drink out of a glass doesn’t appear ocean-blue. This process of light absorption and reflection is also the reason the sky is blue—but the blue color of the ocean is not because it’s reflecting the color of the sky, as many people believe.