I always wondered about this, how deep is the water beneath a water fall? I would suspect erosion to erode even rock over time. Are there some visual clues to estimate the water depth? Or is the only safe way to test the depth manually? I always wondered how cliff jumpers or kajakers determine its "safe" to go over said waterfall.
It is completely site dependent, the result of a myriad of factors; the rock type, its structure, the flow of water and how that flow has varied in the past. Flow velocity in the pool below a waterfall might give you some clues as to depth, but the only safe way to know is manual inspection. Even a deep pool might have boulders lurking under the surface.