Something I've had some success with is food rafts. My biggest problem is the predators that eat the ducklings. I found a spot with good overhead cover and anchor a raft there. The Foxes and whatever find it harder to get at the Ducklings while they are feeding.
The population starts to grow over the years from returnees and the survival edge you've given the younguns.
Feeding them may be against the law, it is here, but I do it anyway. I just put my rafts in places that are seldom visited. And I do a decent job of predator control.