Good luck! We have a kid pool about 5 ft across by 8-10" deep. The very first thing they do in the morning when we let them out is head straight to the pool and get in!
I read on a chicken company website that ducks really didn't need a pool to get into, that they were fine without it...I do NOT believe that at all! Not the way our ducks love their pool!! We'd have to cover it to keep them out...and that would be sad!
Good luck with your duckies!