Being a Canadian fat guy, it's not as bad up here in the summer. I did however play college golf in Virginia and a couple tournaments stand out. First week of Sept. in High Point, NC playing Oak Hollow (great early Pete Dye, muni) in 95 degree heat with humidity. That wasn't the worst of it. It was the over 6 hour round that killed us. Standing on the 11th tee for 40 minutes was just too much. The other tournament that stands out was at National Chamionships just north of Oklahoma City (Oak Tree, another Dye, East Course, not the Men's Club unfortunately) near the end of May. It was also 95, but without the humidity. There were the 30-40 mph constant winds to deal with however and that dried you up pretty quickly. Luckily the NCAA had water and Allsport (Pepsi's feeble attempt at sports drinks before they bought Gatorade) at every tee. I don't recall ever having to leak, yet I grabbed a bottle of liquid at just about every tee. I can't say what was worse, but I think golfing with the humidity in the south was a little worse.
I'm one of those guys that think golf carts should be banned except when needed for medical reasons, but remembering trying to golf in Virginia at the end of August leads me to the conclusion that golfing in the summer down south could lead to a medical reason-so ride on during the unbearable days, but try to walk otherwise. It's the only way to see a golf course. The exercise isn't the worst thing either!