I got my three ducklings a bit late--they were already a week old. Based on prior experience with ducklings, I simply use my garage sink. I started using tepid water, but I am starting to think that cold water could be fine. This opinion is based on observing nature; when mama duck takes her string of babies to the pond, the water must be quite cold, and nobody seems to mind it. By now, my little ones are nearly a month old, and can barely fit in the sink! It will be time to use the giant livestock tub. They were excellent swimmers right from the start, and certainly love the water. "Ducking" their heads underwater is a sport to watch! I suppose they are doing it to clear their nostrils, but it sure is fun to watch.
Good luck with your brood.