Ducklings reared by their mother swim with dry plumage a day or two after hatching, but those reared artificially may become wet and waterlogged after only a short time in water.
It was thought that preen oil from the mother helped to make the ducklings waterproof, but recent research has indicated that this is not the case. Clean down is quite water resistant but its simple feather structure means that contaminants can quickly cause the down to stick together and lose its water repellency. The down then becomes waterlogged and the ducklings may get hypothermia or drown.
Because of the risk in captivity, ducklings less than two weeks old are only provided with water in very shallow, pebble-filled bowls or via drinking fountains.
Other useful information
Ducklings in a swimming pool
Mother ducks and their ducklings