When I can't keep their swimming water unfrozen, I don't worry too much about it, I just make sure to keep their drinking water unfrozen. During extended frozen periods, I will make the effort to get the pool filled occasionally. When they've had about an hour of swimming, I'll dump the pool so...
Are you saying your duck's feathers are not repelling water and look wet? If so, I would be concerned about letting them be in the rain when it is so cold out. There's a thread over in the geese section about "wet feather."