The sea is blue primarily because water itself absorbs colors with longer wavelengths, such as red, orange, and yellow, more strongly than colors with shorter wavelengths, like blue and violet. When sunlight, which contains all colors of the visible spectrum, enters the water, the longer wavelengths are absorbed quickly, while the shorter blue wavelengths penetrate deeper and are scattered by the water and small particles. This scattering of blue light makes the water appear blue to our eyes. This effect persists even below the surface, meaning the blue color is not just the reflection of the sky but is due to the water's interaction with light. Other factors like sediments, algae, and phytoplankton can alter the color of water locally, making it green, brown, or other shades.

In summary, the blue color of the sea is due to the selective absorption and scattering of light by water molecules , with blue light being scattered and penetrating deeper, making the ocean appear blue.