Here in Ohio it is SO hot and muggy! I see what you mean, but I don't think I would add anything. I just make sure those water buckets are full, and then when I get home from work at 5:00, I empty them out and fill them up again.
Once when it was a terribly hot summer, I noticed my hens were panting and holding out their wings from their body to help get cooler. That summer, I had saved Cool Whip containers. I used those to make big ice cubes to put in their drinking water. They seemed to like it.
Maybe you could add some electrolites or something if you really think they're suffering from the heat.....