One of the chicken magazines we got said that if they are truly overheated, to use water that is close to their body temperature.  You want to cool them down, not shock them.  The cold water is harder on them than a gradual cool down.  
We've been putting ice cubes in their water, freezing big yogurt tubs or pie tins with water and some chopped up veggies or fruit, and we have been giving them extra produce that is cold or frozen to pick at.  I have been checking their water several times a day, too.  When it gets over 100, we've been putting an EZ Up tent over at least half the tractor to allow them shade when they wouldn't otherwise get it in the afternoon.