Ducks really are VERY hardy. Last week, a fox attacked one of my young males. He had some minor injuries on his back, but something was wrong with his legs. He couldn't put any weight on either leg. I put him in the shade with his siblings, with water in easy reach (and sometimes I would carry him to the bathtub, if he wanted), and let him rest all day. Then I carried him into the night pen for the night. There were NO external leg injuries. I was planning to ask about it on BYC the next day, but when morning came, he was only limping. Now he's fine.
My main male was also attacked a few weeks ago. One leg was sliced open, and his underside was kind of torn up. It only took a few days of resting and he was back to his daily activities!
So far, in my experience, they're pretty good at healing themselves. But swimming will help because then he doesn't have to put much weight on his leg. I have no experience with treating wounds, because they've all just healed on their own.
~Hannah