The appearance of the colors of the sea depends on the brightness of the sky, the concentration of particles suspended in the water, and the position of the observer.
The sea mostly reflects the sky – when the sky is red, the sea does not seem as blue. Also, the colour blue is more easily bent, or refracted, than red light. Thus, the light refracted back from the surface of the sea appears blue.
For deeper seas, the blue color is given by the penetrating sunbeams. When a sunbeam hits the water surface, it doesn’t stop its course at once, but splits into different hues as it goes deeper. The color yellow disappears along the first meters, then the red fades, and finally the green. The only color left is blue.