I let my ducklings swim at just a couple days old. I use nice warm water in a paint roller tray (obviously a new one that's never been used). They get to swim in the small body of water first. I do this once a day till they're getting a bit big for the tray. Then I move up to a larger container. I like the paint tray because it has a natural incline so the babies can walk out of the water easy. The bigger tubs I use, I put bricks in with graduating sizes so they can get out of the water. I don't do long swims at first.
My 2 week old ducklings now go for swims in the bathtub, again with warm water. They dive, swim and play the whole time. I still put a brick stack island in the middle of the tub so they can get out when they wish. I also sprinkle chick starter around when they go for swims. What a huge hit that is. They love sifting for chick starter. I do it over the side of one brick that's just under water but 1/4-1/2" deep. They absolutely love play time. Still once a day.
I haven't started taking our new 4 for swims. Just got them yesterday afternoon. Today will probably be their first swim. I'll start again with the paint roller tray.
Now everytime we do swims I dry the ducklings off as best I can with a towel and put them strait back in their brooder. All this happens inside. We have a room we brood and incubate in so its a warm room anyway. They have done very well with all of this.