Microwave remote sensing missions such as ESA's SMOS are used to compute variables such as soil moisture and sea surface salinity, using the dielectric constant. This is done on large spatial resolutions of +25km.
However, high resolution SAR data is now available from (e.g.) Sentinel 1. While Sentinel 1 data is widely used for determining soil moisture properties, I cannot find any applications in which it was used to determine salinity properties of (coastal-) waters. Little to no literature can be found about this.
Am I missing any obvious reasons (maybe radar-related limitations) why this is not already done?