There is a big difference between a chicken FLOATING calmly while you watch it and actual swimming. Their feet are not made like a ducks, they won't catch the water and propel them. If they get lucky enough to get to the side and have something for traction to get out on they will survive, if they can't get out by standing on something, they will drown. Anything can float for awhile if they don't struggle, if they struggle because they are scared, they saturate their feathers, losing the protective oily coating they have. Then they die screaming and thrashing, and sinking, suffocating, until their lungs fill with water and their brains die, and they follow. Do chickens needs a swimming pool? No they don't. A shallow pan they can stand in, fine, but don't ask a bird that is not meant to swim to do it to satisfy a human's curiousity.
I have dipped my hens (just up to their bellies) in a kiddie pool to cool them off when it's REALLY HOT. It's funny if they do the paddle... paddle... paddle right before they go in.