With my first ducks I waited a while before I introduced them to water because of the oil issues mentioned, but one of them was constipated for the first 3 or 4 days and had weaker legs even with niacin supplements and electrolytes so she was "floated" in a warm bowl of water under tight supervision and gradually moved to the sink, then with the rest of the flock when they got water time at about 10 days or so. When that flock was fully feathered, she was the first in the water and the last out and was the best swimmer out of all of them even though she was small because of her slow start. Also she had better oil on her new feathers and the water slipped off better (the others eventually caught up). Because of this, I started my second batch swimming on day two with a thermometer in the sink. They started in a 100F bath (still cooler than their body temp) and came out when they slowed down a bit or the water cooled to 90F. They seemed very happy and excited about it and both of them had oil on their fuzz well before they had feathers. By two weeks, they were diving for peas under water (100% supervised) and coming up dry. They are also in the most likely swimmer category today and they are runners. I think the biggest considerations are 100% supervision - they can drown in an instant in 6" of water or in 1" of water. Don't fatigue them - ducklings need to eat and sleep frequently and shouldn't be prevented from either for very long. Keep them warm - they can't regulate their temperatures yet so don't give them the opportunity to get cold. Oh, and 100% supervision Edit: when I say day two, that is my day two. They were hatched/shipped Monday, I got them Thursday, they Swam Friday so they were actually 5 days old.