We got our ducklings when it was fairly cool here. So i let them swim in the bathroom sink or tub with the space heater on, and made sure the brooders stayed warm for them. I kept my hand in the tub and would pick them up to let them rest on my flat palm, then they’d hop off when ready. I always gave them water deep enough to touch bottom first bath, then just deep enough so they could barely touch second bath, then deep enough to dive third bath and thereafter. I’ve read that bathing young speeds development of feather waterproofing. But the information is kind of mixed on the subject.
You absolutely have to watch them in the bath because they can drown. You also should dry them pretty well after the bath too.