When it's especially hot, some of my chickens will get into the long, low water container we have for them, so that they *can* get in if they want, but can easily get back out again. Some of them always seem to enjoy the activity, and others look at them like they've grown a duck bill. 
But, as those above have said, chickens are NOT natural swimmers. I've lost chickens before who managed to wander out to where the horse's water is, and jumped or fell in, and couldn't get back out again, as the water was a few inches below the rim, rather than up to the rim. It was quite sad, but nothing I could have done... it's not as if I could have paid someone to watch the horse's water trough all day, every day.
But, as those above have said, chickens are NOT natural swimmers. I've lost chickens before who managed to wander out to where the horse's water is, and jumped or fell in, and couldn't get back out again, as the water was a few inches below the rim, rather than up to the rim. It was quite sad, but nothing I could have done... it's not as if I could have paid someone to watch the horse's water trough all day, every day.
