Why deep water looks blue
The ocean’s color comes mainly from what happens to sunlight inside the water, not from the sky reflected on its surface. Sunlight contains many colors. As it travels through water, water molecules absorb some wavelengths more readily than others—especially the longer red, orange, and yellow wavelengths. Blue light is absorbed less quickly, so it can travel farther and is more likely to be scattered back toward an observer.
Scattering redirects light in different directions. In clear water, the combination of selective absorption and scattering leaves more blue light reaching your eyes. Water itself has a faint blue tint for the same reason; across a deep ocean, that small effect becomes visible because light passes through a great deal of water.
Why the color changes from place to place
The effect depends on how much water the light crosses and what is suspended in it. A shallow patch over pale sand can look turquoise because sunlight reaches the bottom and reflected light returns through the water. Tiny particles, algae, and other material can scatter or absorb different colors, making water appear green, brown, or cloudy. In very deep water, little light returns at all, so it can look nearly black.
The sky can still affect the view: a calm surface may reflect some of its color, especially at a low viewing angle. But that reflection is only part of the picture. The blue of a clear, deep ocean is chiefly the color water leaves behind after absorbing more of the other colors.
