Mr. Son vs Youtube

My comment:
If you start to feel ill drinking just water -- START to feel ill -- switch to something with salt and sugar in it. Gatorade is better than soda, but even a Mountain Dew is better than your electrolytes crashing. I've absolutely made myself sick being out in the sun in 100f+ temps with only water to drink for 2 hours.
ETA: Two hours of walking in the hot sun for me equals about 5-7 20oz Gatorades. You need to replace a LOT of fluids and electrolytes if you have to be outdoors in the heat.

Wet cloth on the back of your neck. Not ice cold, just out of the tap cool is fine. There's lots of blood vessels back there and you can leech heat out of them pretty fast.