The short answer Baby Ruth is they can't swim so will drown quickly. The reason is their feathering, which doesn't contain enough oil for submersion because they can't produce enough. So it's like you trying to swim in Winter clothes.
I did hear a very sad story once of a chicken used as a broody to hatch ducks. It said that she was successful and when then ran down to the pond and jumped in she tried to rescue them and drowned in the process. Now I'm not sure that's a true story because duck chicks aren't waterproof either until about 6 weeks. They rely on their mother to oil them from her gland until they are old enough to produce enough oil of their own. As I've just said, chickens don't produce much oil for waterproofing themselves, let alone duck chicks. I had a duck chick rescued by a passer-by from drowning in the canal. She was soaked to the skin and freezing cold so I warmed her in cupped hands for a few hours and she recovered, so fed her chick crumb with water. 'Lucky Duck' we called her and at 12 weeks she was flying around the house and then she left us, returning a month or so later with a boyfriend- how sweet is that!