Oceans, lakes and rivers appear dark blue in the Day Microphysics RGB images.
Water acts almost as a black body for short wave solar radiation, hence the albedo is near zero and there is no contribution from the red and green colour beam.
The blue colour beam reflects the water temperature. Water not covered by ice contributes only little with blue colour, but since the red and the green contribution are almost zero, the overall colour hue is dark blue.
The image below shows the eastern Mediterranean Sea early March 2013. Only a few clouds cover the sea surface.
Explanation of the dark blue colour of oceans, lakes and rivers in the Day Microphysics RGB (see also the recipe on the left side):
• The dark blue colour of oceans, lakes and rivers is formed by a moderate contribution of IR10.8 only.