Use the Land/Sea Mask operator - you will find instructions here:
- SNAP: Clip Raster by Shapefile
- How do creat subset image by vector polygon using S1-toolbox?
- Clip raster file using Shapefile
- Subset using Vector file
- How do creat subset image by vector polygon using S1-toolbox?
- SNAP: Clip Raster by Shapefile
- How to Subset a Image in the shape of Polygon from a geocoded Image