I haven't raised those exact types of waterfowl but I've raised others and one thing I can tell you as that waterfowl are bulletproof in terms of tolerance for dirty water and food. I wouldn't worry about it at all. It is well known that waterfowl very rarely get sick from dirty conditions. I sure wouldn't worry about it unless someone else tells you that the swans are not like other waterfowl and are particularly vulnerable. If you are really worried about it find a way to put slats over your feeder so that the birds can only reach their heads into the feeder and not their whole bodies. That's what we do with chickens since they are somewhat sensitive.

Gotta get back to fishin!