I let my ducklings swim from day 3 on. That is in warm water with supervision. I usually start off in a paint tray and go deeper as they get older. The key is to towel dry them off and put them back under the brooder to dry. You will immediately notice that they are preening themselves, even though the gland is not producing the oil yet. This daily exposure to water actually does promotes the oil gland and they will sooner produce then those that are not exposed to water. They get unsupervised swimming outdoors once they are 3 weeks old and it is warm enough. I start off with low Rubbermaid storage plastic tubs (4" deep I think). Don't forget to put something outside to climb in and most important something inside of it so they can get out easily. A brig will usually do the trick. Then you can gradually go deeper as they go. My 8 week old ones are now in a 10" deep kiddie pool.