I leave a sprinkler on, which they LOVE to stand in and drink from, or a hose trickling at all times. They seem to prefer the puddles of water to swim in AND drink from, rather than the kiddie pools, though they'll go in everything wet.
I think they prefer the puddles because they can do their rooting around with their beaks and make nice little golf-ball sized holes in my yard.
I dug out enough ground under all of my citrus trees and a date palm so I can make a mini-lake and have the tree irrigated at the same time. The ducks made the holes a little deeper and more defined. But all of my dirt walls that I made around the holes... wasted effort. They dug through with their beaks and crumbled the walls in no time.