At least a foot mine did fine with a kiddie pool. But what is neat about my setup is I have a small kiddie pool about a foot and a half deep that runs through their pen that makes a little stream and runs out down the hill next to the pen and they really spent more time in that just wadding in it more than swimming, but either way let a little water run out of their pool and they will dabble and play in it looking for food, since they are natural dabblers and it is their favorite thing to do. 
