When to apply Mask in S1-Workflow

the Land/Sea-Mast operator does what you need, and you can include the shapefile in the graph as described here: Error in graph builder for subset image through a shape file

But please be aware that this only removes the pixel values outside the vector. The size of the raster will stay the same.

May I ask what the first Write operator is for? I think you can directly connect Subset to Calibration.