Ducklings need a waterer they cannot sit in at all. They are just doing what they think they should do. The mother duck would take them out on the water but they can crawl up on her back and sit when they get tired. They also need to have their food away from their waterer. That way they develop their legs faster. I use a standard chicken waterer and they can play in that without getting wet.
I do not medicate my chicks or ducks. If I have to medicate my chicks for any reason I remove the ducklings. I also do not house my ducklings with my chicks longer than it takes for the duck to learn to play in the water.
It is harmful for the chick to be so wet. Ducklings can survive outside when their DOWN has come in, or when you see the pin feathers start. I use a old dog house they can go in when they are needing protection from the elements.
Ducklings need a small amount of play in water DAILY, but I do NOT put a pool in the pen, for unsupervised swimming, till they have their belly feathers in. Then I put in a small kiddy pool with water in it as then they can float. You will need to put up a step they can get in and out of on. They do however quickly learn to lunge out of the water, so only a brick to help them get in is needed.
If you are planning on have a lot of ducks, you can get sometimes free old truck canopies and use them in the field for shelter. Putting hay in it should ensure that they could use IT instead of isolated nesting sites making it safer. I would suggest applying sevin dust to the earth before hay is put down to discourage mites and other bugs. My geese have finally decided that the canopy I have had down for them, for two years, is a great place to have their nest. Now that they know that, I can move and when I put it down in the new place, they will know what that is for. It is very prudent to put the canopy UNDER a tree that has tons of leaves. They can get HOT under the canopy and be sure the windows are open...
Once ducks are fully grown (6mo) and are fletched out they don't need more than hen scratch each day as a meal. They will eat what they need making up for the protein in their diet by drumming out worms or grubs in the field. You can however give them layer pellets a couple of times a week if you feel they need it. But my ducks are highly productive on what I give them. Geese need extra as they grow for two years, and need the extra protein but not too much. Geese should have turkey starter till fully fletched out and then reduced to layer pellets for the rest of their growth. They are very effecient foragers.
Hope this helps.
Arklady