To get a better understanding of the variation of backscatter intensity in water bodies you can digitize their surface and analyze their statistics with the statistics tool, e.g. to find their 90 percentile.
Described here: Ellipse drawing tools
And here: Classification Sentinel-1 problems with MaxVer
There are many topics and diso on water body delineation in here, you might want to have a look at some of them to compare different approaches.