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.
Explanation of the colours of the open water surfaces (see also the recipe on the left side):
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.