This photo taken in Durdle Door, Dorset in August 2009. A bank of cloud cover can be seen on the left of the photo resulting in the sea looking grey. On the right of the photo there is blue sky and the sea looks blue. This gives a strong indication that the sea, at least at a significant distance from the observer reflects the colour of the sky.
As an aside, the blue colour of the sky is due to Rayleigh scattering. As light travels through the atmosphere, most of the longer wavelengths pass straight through. Very little of the red, orange and yellow light is affected by the air. Much more of the blue (shorter wavelength) light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.
© 2016 Dr Matthew French All rights reserved.