I usually do a mix of modeled geometry and textures. For modeling there are a few different approaches. But generally after getting faces in the basic shape you want the water fall to take, then subividing to allow more detail - Artisan, Vertex Tools & SubD will get a nice shape going. Winning with Sketchup has a cool youtube tutorial on using Eneroth's Fractal Terrain Eroder to get water shapes, also.
With Vray, I'll make a copy of the pool water, then change the uv tiling in the vray materal to be stretched in the direction the water is falling. This makes the water look as if it is falling. I use Interactive render with a small render region to help speed up previews.
Also, tend to adjust the refraction intensity/IOR, as well as the reflection amount to get things looking right.
And of course....always have a nice hi-resolution inspiration pic for reference!