I guess it would depend on the depth and color of the sky at that time to depend on which would be darker. In other words you both would be right. Correct?!?
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The sea is blue for the same reason that the sky is blue:
In the first place, the sky is blue because the molecules in the atmosphere scatrter the blue part of the spectrum, and the red and yellow parts of the spectrum tend to pass straight through. What happens is that the blue light is absorbed by the molecules in the air; then 're-emitted'. The re-emitted light may get absorbed and emitted many times over before it reaches us. To us looking from the ground, the blue light seems to be coming from everywhere (this is called 'Rayleigh cattereing'); hence the sky seems blue.
The reason why the sea seems blue is similar; but a little more complicated. The sea does reflect the blue colour of the sky (or can appear grey in overcast weather), but this only applies to the surface of the sea. As well as reflecting the blue from the sky, the water molecules scatter the blue light in much the same way as the air molecules in the sky. This accounts for the 'natural faint blue colour'.
In addition, the water also absorbs more of the red and yellow wavelengths than the sky (which warm the sea), leaving the blue part of the spectrum, as well as part of the green. This is why deep water can appear bluish-green. (The absorption of the red wavelengths can be noticed by looking at a red object under deep water. It will appear black; because only blue light is reflected back to us.)
Eventually, at sufficient depth, even the blue light will have been absorbed or scattered away, and deep water will be black and lightless.
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