Manufacturer says:

Q: Can I use your faux wall panels in a pond or pool, or run a waterfall over it?
A: Our panels are used in many ponds, waterfalls and are even safe for fish! (As substances like chlorine and other minerals may damage the finish of the simulated siding, please note that the warranty for this use does not cover the finish.) Texture PlusŪ suggests ordering a free sample and testing it out first.
I want this for behind my waterfall. Looks like it might work.

Wanted to post this in case anybody else might be looking for something like this.