Thanks @Skulburn, I’m really interested in how it works.
I’m still wondering about height-map usage. In my case I’m not using landscape, but imported geometry and I have multiple islands, each being a seperate mesh and some are even floating (not connected to the underwater-ground).
Would the best method be to combine the islands in an external editor, create a heightmap of the whole scene and use that as some kind of a mask on my ocean plane?