The sea reflects the sky. The sea is not very blue when the sky is overcast. Water reflects and scatters the light that strikes it. This is shown by the fact that we can see our reflection in puddles. When the sky is brilliant blue, the sea is also, because it reflects the blue of the sky. However, the sea is still blue though not so brilliant even if the sky is gray.
The second reason is exactly the same reason on why the sky is blue. Blue light is more easily bent, or refracted, than red light. Thus, light refracted back from the surface of the sea appears blue. Furthermore, when, underwater the water around appears blue because more blue light is scattered back to your eye than red light.
The deeper the sea is, the bluer it becomes. 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.
Shallow waters on the other hand can only give off a light blue and sometimes even green but deeper waters give off a deeper blue color and if you go diving deep enough, you will be surprised that the surrounding color turns black as sunlight can no longer penetrate it.
Done by Jeremy