I built my waterfall in the corner of two walls I did not want to risk getting any moisture on.
Here's how I did mine.
First I made sure I had enough primary pond liner to pull up the corner between the waterfall structure and the wall.
You can see the excess liner in this photo slumping down in the corner.
In this next picture you can see the the blocks in place for the waterfall structure, and some of the liner pulled up between the blocks and the wall.
The liner is not directly against the wall, I have styrofoam with grooves cut out one side placed between the liner and the wall. The grooves act as air breather spaces against the wall to allow any trapped moisture to escape. (Moisture from rain, not the waterfall.)
This next photo shows the secondary waterfall liner in place. I then folded or cut back the secondary liner to create a gap between that liner and the primary liner (mainly on the left side), then mortared rocks in place along the stepped area and created a flow channel for the waterfall.
I also used scrap pieces of liner under the blocks to protect the primary liner from the rough edges of the blocks, that's why you see so many layers of liner in the pictures.
After the mortar set I ran the water and tested and re-tested the flow of the water by creating dams and diversions with rocks, with some difficulty it was possible to get the water to seep over the edge of the secondary waterfall liner, but in ever case it trickled back inside the main liner and into the main pond water. After I was sure this double liner system worked I continued placing the surrounding rocks to hide all the blocks, liner and gaps.
With this double protection system I don't have to worry much about the plants blocking the water flow, I'm pretty confident that even if the plants do cause the water to divert over the waterfall a bit, it will flow back inside the primary pond liner.