Its very true and very scary how quick heat can hit you, even when trying to stay hydrated. I used to be active in a medevil re-creation group (SCA) here in Florida. Wearing about 50lbs in metal and hardened leather armour was my norm...
One event when it was in the high 90's, pretty much anytime April thru September, I came down with heat exhaustion. We have water bearers who keep the guys as hydrated as possible, squirting water into our mouth via a sprayer and also will shoot water on us (usually the chest/neck area). It was a long list, and a brutal one, no elimination just keep fighting rotating between 6 rings. Fight until you're killed, get back in line, repeat.
I was being careful to drink whenever given the opportunity, until my belly felt full. I knew by the time I hit the pits I would sweat/absorb enough to not be bloated. Well, after about 2 hours into this three hour grind it hit me. Just standing there waiting when all of a sudden I felt this wave of heat roll out from my padded jacket, up into my helmet. I instantly felt my knees go weak and felt sick to my stomach. I dropped my sword and shield on the spot. Started walking slowly and carefully to a table that was about 25' feet away removing my helmet and just letting it hit the ground. Called a water bearer over. Yelled for a chirurgeon (first aid responder) because being one myself I knew how bad this was.
I went through 3 gallons of water, drinking between a 1/4 and 1/3 of it. The rest was drooled down my chest, sprayed over my back (as soon as I was stripped from my padded jacket and armoured belt),my head my armpits and my crotch. I also had 2 quarts of gatoraid fetched from my camp (for the electrolyte replacement). All in all it took about 45 miniutes of doing NOTHING but sitting in the shade, drinking to rehydrate, being doused in water and having three fair maidens (god bless them) fan me to get back to normal. A very weak and ehausted normal, but out of the danger zone.