I agree with jan. Rocks are ok for the ledges, but I strongly suggest you keep the bottom clear.

Waterfalls are the main source of leaks. So be particular when you construct this. Take your time. I used greatstuff foam for under my waterfall rocks. I do not want a waterfall that is too permanent- I might want to change it later, so no mortar for me.

I have heard of a product that is supposedly better than silicone for the skimmer face plate and for attaching the liner to the bio-falls. It is called Sikaflex 291 and cures under water. Told it is sold at many home improvement stores.

Good luck with your new pond. Happy ponding!