My pond is not fence either. We asked the people we bought them off of how to get them to stay. They said just put them on the pond, make sure they have food and they will stay. So we did and they did, although they had a little bit of wander lust at first, walking around the pond and into the yard, but never more than about 100 yards away.
Yes it was a good deal and in my opinion it was the best $250 we ever spent on birds. We usually get really bad algae blooms in the summer and since we had it dug out a few years ago we started getting milfoil. No matter what we did we couldn't get rid of the milfoil and we constantly had to dump algaecide into the pond to keep it controlled, but it always came back. Now with the swans they eat take care of all the milfoil and algae for us, it is great.