How to use the Water plugin with imported terrain?

Hello,

I am currently trying to use the new Water System with an imported height map.
For this, I would like to add water, that does not influence the heightmap of my terrain, but instead works similar to how an oldschool water plane would work, but still being able to use the splines to form the shape of the water surface.
I would also like to use the Ocean System with imported terrains.

Unfortunately I could not find anything helpful in the documentation.

Thank you very much!