I have incubated one duckling from an egg and started introducing it to water since it was somewhere around 7 days old. In just a day or two, I have noticed it started to apply oil from the gland above it's tail. It was still completely fluffy without any real feathers.
Then I bought 3 more ducklings to keep him happy with a good company. They had no working glands and were 4 weeks old, after a few days, they also started swimming and taking baths, they oiled-up really fast.
Then at around 9 weeks, I got 3 more ducklings. They were feathered up OK, but they got wet as soon as they met the water. They were not used to water and didn't care about waterproofing their feathers. They did that in a day. Took a couple of swims and a lot of dedication to their feathery coat, but the following day, they were all dry when they left the pond.
I also had issues when I got the last 3 ducklings. The remaining 2 drakes were attacking them (not too hard, just scaring them, not physically assaulting them) as they tried to enter the pond, got near food or just when they wanted to lie down next to them. I closed them all into the coop at night and there was no noise coming out, so I expected them to be OK. The following 2 days, there were occasional incidents when the two drakes used their bills to scare the three newcomers, but they eventually let them join the pond and they were no longer bitchy around the food. After 4 days, they were friends and the larger drake started acting as a group leader, defending the flock. All the other ducks acted accordingly, they respected the big mofo.
Don't worry if they don't start swimming immediately. They need to adapt first. It took my duckling quite some time to become accustomed to water. At first I picked him up and placed him into the water. At first he jumped right out, but after a few days, he became much braver and even started swimming. By his third week, he rather stayed in the pond than outside. He would stay in the water during the night as well if I wouldn't make him go into the coop.
PS: it's interesting how fast the drake responds. When the sun goes down, smaller ducks start to make noise and call the group to go to the coop (they learned that in 2 days!) and he instantly runs towards them and leads them inside.
