I'm sorry to contradict WYSIWYG, but do NOT wait until the little fella is fully feathered before letting it swim. We've had permanent leg problems in all sorts of waterfowl raised without swimming EARLY ON. It should be able to swim if it can walk; the Momma ducks take them to the water the day the last duckling hatches. But don't let it swim unsupervised, cuz they can get themselves into trouble (get tired, or get where they can't get out by themselves). And if he does get wet "to the skin" (even the fuzzy stuff should be fairly water repellant) by all means help him dry off and keep him warm until he is.

Any other questions feel free to ask me, too.

campfire (zookeeper, with more bird-raising experience than I care to admit)