Sea, lakes, rivers (open water surfaces) appears dark blue in the Snow RGB images.
See for example the Mediterranean Sea, Black Sea, Baltic Sea and North Sea in the image below.
The next image shows a closer look on the Black Sea and the Dnieper River.
Explanation of the colours for snow covered land (see also the recipe on the left side):
• The reflectivity is very low in all three channels.
• After performing the linear stretching and the gamma correction the signals are still very low in all three channels. In most of the cases the blue component is slightly higher that the green and red components, so the open water surfaces appear dark blue, sometimes almost black.