For water bodies it shouldn’t make any difference because they don’t underlie topographic variation.
There is a relationship between the incidence angle and backscatter bias but this applies to slopes towards/away from the sensor only.
If your water areas have steep coasts there could be some shadow or foreshortening depending on the flight direction, but for Sentinel-1 this would make only a pixel (on the land).
Very steep coasts at 100 or more meters above the sea level could indeed cause problems for the exact discrimination between water and land.
This could be interesting to you:
Relationship of local incidence angle with satellite radar backscatter for different surface conditions