Where are you?
I'm not good on the duck knowledge, but around here, we have lots of coyotes and they're the least of my worries with my swans. The biggest concerns are the minks, weasels, and snapping turtles!! Once when I was freaked about the coyote that was in my yard the previous evening, I called our local Fish & Wildlife guy (long time ago when the swans were relatively new to us) asking if he could help. His answer to me was, "Beth, I've seen your male swan--and I'd be more worried for the coyote than for him!"
Yes, our Samson is really that much of a brute! But the way I've learned that minks kill their avian prey--well, that scares the you-know-what out of me, not to mention the thought of a swan/cygnet losing a foot or head to a snapper! Unless someone has a better idea, the only thing you can do for those problems is have a great trapper who's available immediately when you need him! I've found that the little old man who does this for me is far more effective than I would be trying to do it myself, especially when time is of the essence.