Interesting question! While the sky and ocean may both appear blue to our eyes, they have slightly different shades of blue due to the way light interacts with water and air. The ocean appears blue because water absorbs colors in the red part of the light spectrum, making the blue light more visible. The sky, on the other hand, appears blue because of the scattering of sunlight by the Earth's atmosphere.

So, to distinguish between the two using only blue, one could observe the nuances in the shades of blue and try to determine whether it looks more like the color of water or the color of the sky. However, it would be more accurate to use other colors or contextual clues to differentiate between the two.