As Thierry said, you basically already have all the things you need for a waterfall.
First, go into object maker. Click the Mesh tab, and type in Waterfall1.mesh .
Click the "add new" button in the bottom left of the panel. The material should come up with it.
From there, you give it a name, save it to a file, and make yourself a little funtimes ceiling waterfall.
Tip: If you place it in it's own custom folder (and not, like, 'myrocks' or something), and place all other custom content in the same folder for that map, FH will export the object file for the map when you distribute it. That way, people don't have to edit their own object files.
Hope that fixes your problem. ^^