I have considered that but it would take an awful lot of rocks, I use those black rubber feeders for waterers and I have about 25 of them, I likely will just make a crap load of maintenance work for me to tend to when I can just take the chicks from the few mama birds who are terrible about leading chicks to safe watering stations.
I clean my watering bowls twice a week, its a lot of work when I have 25 of them, if I included rocks in them it would take all day just cleaning the algae and poop slime off the rocks too. For now it takes me about 40 minutes to clean them all and I really do not want to spend more time than that.