[enriquto]

The liquid water absorbs most of the radar wave (at least for the C band, as is the case of sentinel-1). Thus, in the images it appears natively "black". When there are a lot of waves, the surface of the water forms some spurious reflectors that appear as a light texture in the surface, but the signal is definitely less powerful than metal/concrete reflectors of buildings and ships. Notice that in the "Humber" image of TFA you can appreciate some texture in the water. The contrast in this image has been exaggerated a lot, which saturates most of the land.

[m2fkxy]

It actually does not absorb radar energy, rather it reflects it away (specular reflection). That's why rougher sea surface appears brighter on SAR as it turns from a specular to a diffuse scatterer. Furthermore, in some specific conditions (low incidence angle, closer to nadir), calm water can appear much brighter than other land surfaces.

[enriquto]

thanks for the clarification! I'm just used to looking to the images without thinking too much about the water.

I've never seen "close to nadir" radar images... wouldn't the ground fold over itself?

[m2fkxy]

Nadir-SAR would look very confusing, full of ambiguities, and probably unexploitable since the ranging part of RADAR would not be able to distinguish the left returns from the right returns -- that's why SAR is side-looking.

even with a strictly side-looking geometry, images collected closer to nadir (steep/low incidence) start exhibiting some artifacts such as nadir returns [1].

[1] https://www.researchgate.net/figure/Example-of-nadir-echo-in...