try setting the water on a brick, don't use a cinder block untill they are much older--chicks and get stuck and die in the cinder block holes. Oh, and with waterfowl the water is always dirty. Can't wait to see the pics.
yes, the big danger is getting chilled, the down has no insulation value and they can't regulate their body temperature yet, but give them 4 weeks and everything changes. It would be nice if it worked for children as well !
I can't provide ID's, but to your idea of letting the duck/goslings swim- they need to be almost or fully feather 1st. Your wanting on a functional oil gland, since Momma duck can't spread her oils on them. How do you know when the oil gland is working? after 4 to 6 weeks when they play in water...