Chickens are pretty instinctual; they know (more of less) what they need. If it were me, I'd give them access too dry loose dirt in the shade to cool off in as well as the water tub, they'll choose the one they need and it'll give them extra water just in case.
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.
Looks like I need to add a kiddie pool to the list. This year the weather has been really strange. It's 100 one day and highs in the 50's a few days later. We've had a very cool summer so far this year, except for these 100 temps heat wave twice this year.
I see you are from MN also, Yesterday in anticipation of the high heat index we went to fleet farm and purchased a 2'x3' tray with 1 inch sides and just set it out there with the hose letting a slow flow of water in for them to walk in and cool down with, I also filled several ice cream pails 3/4 of the way full with water and froze them so that I can put ice blocks out there for them to peck at or cool down with as they desire... seems to be working well for most of the chickens, though we do have a few hens that wont come out of the coop and just sit inside and pant in the heat.... Ugh those stubborn ladies!