First year having ducks for me, bought a couple Rouen ducklings back in mid February. They ate ragged jack kale, they REALLY liked lacinato, garden peas, and cowpeas. But once those were tall enough the ducks couldn't get to leaves and pods, they enjoyed hanging out underneath them on hot days. They'd nibble on pepper flowers and leaves, and corn shoots early in the season, but they didn't bother snap beans, squash, tomatoes, eggplant, carrots, potatoes, or onions. They didn't seem to bother flowers much either- ignored the wildflowers and zinnias, munched on some canna leaves early in the season- but as with the peas, once they were taller than a foot, the ducks couldn't reach them. And typically they'd have to be working at it all day to do significant damage- if I let them out for a couple hours or a half-day while I'm working on the garden, or grilling, or reading, and can keep an eye on them, they're not too tough on things.
I got one of those short (maybe 2 feet tall) wire rolls of decorative fence at the hardware store to separate the front half of the garden (vegetable plots) from the back half (coop, berry bushes, mushroom beds/logs, and a couple overhanging trees. That mostly worked- Like Saybian said, if they find a flexible spot they can sort of barrel their way over. But they're easy enough to herd back in, and you can bend the fence back in place and reinforce it with stakes as needed.