Ms Lydia gives some of the best advice you can hope to get. But, usually ducks are not ready for unsupervised swim time until 8-10 weeks of age minimum. Until they have their adult feathers no more than 5-10 minutes in the water no matter how they seem to enjoy it and then only if it is warm or they have a warm place to go and dry out. Ducks love water but this experience might stay with them so don't be too surprised if they seem afraid of water for a while. Make sure they have an easy way to get in and out of the water, you can use bricks or stones to gradually get to the edge of the pool. I too do not know what happened to make them loose their down but it sound like you have it under control now. Just keep them warm and give them a lot of attention. I am glad they are improving.