Hi Christian,
So you are getting there!!
Now that I see what you have in mind I realise that you are talking more about an 'artificial' waterfall rather than a natural stream waterfall. I live in a hilly rural area, with natural streams (when it rains - we have had nearly 5 months without any rain at all until a couple of days ago) and many homes have natural waterfalls etc, so I automatically had that sort of thing in mind.
If you are planning a waterfall over a 'manmade' wall then you are on the right track and there are many ways to do it. You do not need to worry about the Terrain tool if the ground is essentially flat and the waterfall 'above ground level' as in your image.
The waterfall itself could be over a Wall (Any height and width but only parallel sides - curves possible and different textures on each surface), a piece of Floor (ANY shape and thickness is possible with curves and different textures on surfaces), Pathways (any thinkness and curves but only parallel sides like a wall, but only one texture) or simple a 'box' (rectangular only - See Useful Shapes... post
http://forums.turbofloorplan.com/index.php/topic,315.0.html).
The Fill tool works differently, as it only sits on the terrain and follows it. Very useful if you are shaping the terrain itself or for water and garden beds. It does not have any thinkness but, like a floor, it can be any shape you want, including curved edges. It cannot be elevated unless the surrounding terrain is, so if you overlap different fill areas the different textures may interact with each other.
I don't know whether those comments help at all but it looks as though you are well on the way to achieving what you want. If you need more help or suggestions just ask.
Just out of interest I did another render with different water and did Advanced render to see how it would handle the shape of the Terrain. Shows it much smoother than the first image (Basic render). By the way, if you render and use imported Photoboards of trees etc (which mine are) you must set the Water texture to Dull. By default it is set to partially shiny (or Liquid) and it causes reflections on the Photoboards. A bug presulably. If you have imported yourown water texture it probably would have defaulted to dull anyway so shouldn't be a problem.
Will look forward to see your project develops.
Allan