Is it possible to have the post-processor water appear in the SceneBuilder?

The title says it all. I need to be able to do this to position the imported terrains properly with the water level (ie. for islands).

You can create a .j3f and add a water filter to it. Then open the filter in the filter exploring (usually by double clicking the .j3f) and then turning on the eyeball in the scene composer.

You should be able to edit the water to your liking