My ducks will always be enclosed. Right now they are in a garden wagon that I am using as a brooder, it has a lid to protect them from predators and I can move it into the barn at night or on rainy days. In 2-3 weeks, I will move them to a 8ft x 8ft pen (like a chicken tractor, without nest boxes). When they are 8 weeks old, I will be finished with their permanent home, 24ft x 60ft fenced and netted yard. I am going to build a 8x16ft pond 8-20 inches deep for them that I will drain/fill every week. They should be protected from pretty much all predators both sky and ground. I am going to build a 4x8ft 3 sided shelter for mild protection and a 4x8ft enclosed house, for complete enclosure and nesting.
My plan (unless someone's advice changes it) is to let the blind one learn to go to the food and water based on the sounds of the others like he is doing now. My only concern right now is when they are older, and the pond is complete, will it be able to get out of the pond safely. If it is around when I am building the pond, I will take that into consideration when I build it.
Right now, I have too many ducks, I figured they are like chickens and could plan on a percentage of loss. I am not thrilled about the Mallards and I will probably list them on Craigslist when they are around 8 weeks old. If I don't have any losses, that would leave me with 8 and that is what my goal was.