My ducklings were 3 days old when I put them in the tub. Personally I think it benefits the ducklings to learn good bathing and preening habits early. They were put in warm water and supervised the whole time. Then when they had been in about 10 minutes I noticed they weren't splashing as much so I took them out and dried them with a towel and then put them in their brooder and used the hair dryer on low to finish drying them off. They never became chilled and really liked the water. I see so many people say wait until they have feathers but I think it's healthier for them to be in water. Plus it keeps them hydrated and they can definitely make sure their eyes and nares are cleared out by playing in the water. In fact with my weak and sick duckling, I think being in water actually helped her more because she wasn't drinking but she would dabble in the tub and rinse her nares that started to get crusty.
But to each their own.