This has been my first summer having ducks and they (3) free range and have free access to my garden. Yes they will destroy some things. I didn't have many zucchini or yellow squash this year because they thought the leaves were very tasty and they ate most tomatoes that they could reach. I had to put a temporary makeshift fence around some of the veggies that they wanted. My pekins graze continually and I am still discovering their favorite snacks. They just LOVE begonia flowers and leaves. Had to put all my potted flowering annuals out of reach but I have trained them to come when I pick a few flowers for them.
My ducks have free access to the garden and they never touched anything other than a squash leaf here and there and they did trample my watermelon plants a bit. For the most part though they didn't hurt anything.