My waterfall is built with natural stone and there's about a 5-ft section where the stones overhang the ledge and sit in the water (there are also a few under the water as well, so no water line tile in that section. They look ok but you can definitely see a waterline on them when the water level drops. I've also noticed that in two spots under the water where the pebbletec contacts the stones, I get growth of calcium carbonate crystals (looks like white fluffy stalactites). They rub right off, but it is interesting as it is the only spot in my pool where that happens. Also, calcium scale (looks like yellow-white mucous but it's solid) develops on any of the mortar surfaces as well as some of the stone surface. The only way to clean that with causing acid damage to the materials is to using a tile cleaning service that practices soda blasting. My waterline tile remains beautiful and scale free but the rocks in water act completely differently in terms of scale.