Well I guess it's possible, but if ducks will eat rocks and sand I don't understand how they are not extinct. All lakes and ponds and creeks have rocks and sand!
It is nice to have gravel under their area with water it does make draining easier. I think pea gravel would be easier on their feet then the big gravels. Sand is a nice bedding material and can be used in either area. I have read people using rocks so they can spray them off but if you don't...