Yellow flag iris (Iris pseudocorus) or Blue Flag irises, or Papyrus (cyperus), would do well there. Also elephant ears (Alocasia or Colocasia and some Xanthosomas), cannas, Calla lilies (Zantedeschia, Aethiopica), Bowles Golden Sedge (Carex elata, "Aurea"), Acorus (Sweetflag. Get the golden one.)and Spiderwort (Tradescantia). "Sweet Kate" is extra nice.
Some lady ferns (Athyriums) are supposed to grow in very moist conditions, but I don't know which ones, and it would depend on how much sun, for they typically like shade.

Equisetum would also like those conditions. I shudder to recommend it because it is one of the most invasive plants known to man and nearly impossible to eradicate once it gets going. But, you might have the absolute perfect place for it, in a solid rock pot! If you choose it, be extremely careful of where you put it and always give full disclosure if you share it with someone. :)

Most of these plants are what most of us would call "flowers", but they will look absolutely natural around a waterfall and will add wonderful, cooling textures and spectacular foliage colors all summer. Most of them are perennial in my zone, but you would probably have to either bring them in for your winter or re-purchase each year.

Thank you very much! This area can very beautiful with a touch of color. I will give them a try!
Albert
Louiville KY.