Yep, that is why they love when you float lettuce, peas etc.. some of mine desecrated a water Lilly in my pond last year, so apparently it's yummy too.Floating greenery is their most favorite "wild" staple food.
OP, ducks are MASSIVE routine animals changes to that, scares them, it takes time to adjust to new places, new things, i switched up a waterer design recently and they balked for a bit, many adults are in this flock and the waterer wasn't new just the method of my setting up. Patience, in time they will learn and come to enjoy the option.