Get those small watermelons or grown them, put them in the freezer. When it's hot take one and crack it open and the chickens will love it! It works with most water veggies or fruits, like cucumber, watermelon, cantaloupe, ect... You can freeze chunks of fruit or cucumbers and put them in their water, they'll peck the frozen chunks plus get nice cool water. Make sure to place water in a shaded area and fill a couple times a day with fresh water. Sometimes I run my water from the hose to make a puddle and they run/splash in it.
In my area it's hot and very humid during the summer, but my chickens don't mind as long as I give them frozen fruits/veggies, fresh water, shade, and fresh cool water puddles.
Do not feed them corn, it's known to cause heat within their bodies as the digest it. That's one thing I learned years ago from a friend who's had chickens for 10+ years, that bit of info has always stick with me.