If I am getting it right!
You want to study the flow of water over or under your geometry!
So there won’t be any requirement of subtracting geometries as you might have done in Ansys.
You can just define a bounding box around the geometry and assign the location of the material point outside of your geometry! And the mesh would be generated outside the geometry and within the bounding box.
you can also watch some webinars or tutorials to get a good idea of the platform:
And public projects: