Oceans and lakes
Sea, lakes, rivers (open water surfaces) appear black in the HRV Fog RGB images. See for example the Adriatic Sea, Mediterranean Sea, Baltic Sea, North Sea and several lakes in the images left and below.
Meteosat, SEVIRI HRV Fog RGB for 17 March 2014, 10:25 UTC
Meteosat, SEVIRI HRV Fog RGB for 13 March 2014, 11:40 UTC
Explanation of the colours of the open water surfaces (see the recipe):
The reflectivity is very low in both the NIR1.6 and the HRV channel. After performing the linear stretching the signals are still very low.