Freezing isn't a problem here, so I'm only guessing about it, but there are ways to keep an open area for them. I've been duck hunting in marshes where there was just a small cirlce of water that the ducks used. Some would nest on the ice if it was too crowded!!!
I feed them some deer corn that I just put along the shoreline. The goose eats first, then the ducks finish it off, but not always. They seem to find lots of stuff to eat under the water and along the shoreline. I have no idea what they are getting from the mud, but the sure seem to enjoy it. If they see a spider on top of the water, the go crazy to get it. At first, I had thousands of spiders, but after a month, I couldn't find any.
You might want to check my spelling, but I think its right. You can easily buy them as ducklings or even the eggs if you want to go that route. I didn't want allot of ducks running around, so to control there levels, I just bought drakes. Babies are cute for about a month, then they turn into pooping machines. I'm hopeing the four ducks won't cause too much trouble in a 3/4 acre pond.
If you do a search here, there were two guys that had a fairly long lasting thread about there rouen ducks. It was a few years ago and I forget who it was, but it was very informative and intereting. The thread wasn't about rouens, but ended up turning into it. That's when I first learned of them.
They are meat ducks, which means they are too large to fly. You don't have to worry about them leaving, but they look real close to wild mallards. For me, they are perfect.
Eddie