Agua fresca...basically water with cucumber and lime and maybe some other things (there are many ways to make it). A guy I used to work with made an awesome version of it that really hit the spot after hours of physical labor.
And no, the cucumber lime Gatorade doesn't come close to freshly made with actual fruit/veggies.
After working outside on a hot day, water for sure.
Nothing beats a bottled light beer that's been sitting on ice though, but when it's too hot beer just doesn't cut it. Usually I'm one and done and go water.