Please make sure the PB uses a proper waterfall pump for the application. Too often PB's try to just slap on another pool pump to the build and pool pumps are high head (pressure) pumps while waterfall pumps are designed for open-pipe (low head), high flow rate applications. Take it from my experience where our pool's waterfall was installed with a GIANT 3HP Pentair WhisperFlo pump...it is way too powerful and I had to add extra shutoff valves to it to create enough back pressure to keep the pump from cavitating. Also, don't be talked into just splitting off some flow from the main pool pump to make the waterfall happen. If they do that, more often then not they have to rob so much water flow from the pool pump that very little water goes through the returns. The best design is a separate set of wall intakes to a properly sized waterfall pump with all the return water sent to the waterfall. They can even add an additional wall return in case you want to split the flow from the waterfall to a wall return to get the exact amount of splashiness and sound you like.
As for the rocks being sealed, I would agree with the recommendation to wait BUT, honestly, I have had my waterfall for years now with a salt pool and the stones are unsealed and there is no issue.
