First off --- if you are building a waterfall, pond, stream, rock in a watery area, you need to take into account that erosion may expose your facade. I have seen it before where people have tried to incorporate a pond off of a stream bed and they did not take into account that certain times of the year the water levels of the stream are more violent then others. Water is a very powerful tool ( Look at the Grand Canyon )
I would know the area well and make sure that there are no severe changes.
As for water falls --- if you are recirculating water into a pond that works great but just the opposite can happen. If you place your water fall to far away from the natural water edge then the very waterfall you are creating will erode the base.
There are acrylic modifiers that can be added to "water proof" or make the concrete water tight. Call any of the manufactures of cement products. South Africa may be a challenge... Ask Irne.
Or you can put a membrane (rubber) under the cement to act as a barrier.