I cant think of any way ducks could evade minks on the pond, or anywhere for that matter, but you might think of building a dock-like structure that extends a little over the pond. That way, they can be on the pond (escaping land predation) and be shielded (escaping air predators). Yes, most predation probably won't happen in the daylight, so the main thing is to lock them up at night. Make sure there aren't any little holes in their coop (weasels and minks can get through amazingly small crevices; seem to be deflatable) and make sure the door latch is not incredibly simple (something a coon probably couldn't figure out.).