Several thoughts to keep you safe and hydrated. I've learned this over the course of years riding in the desert and during the hot days. First, I wear full leathers, including full finger gloves, with a long sleeve shirt that I've soaked in water and then wrung out real well. If you don't your skin will get sore. I do this every time I stop which is about every hour while I'm in the heat.
Once you're on the bike and moving down the road, the evaporation will be just enough to keep ya pretty darn cool. Second, I agree, leave the full face helmet at home, it will only get hot and believe it or not, you brain can swell causing other issues that you don't need to contend with. I use a half helmet. Third, I cover my face with a thin nylon pull over that keeps the wind/sun from my skin. And as mentioned, I drink lots of water and Gatorade to keep hydrated. Just my .02 worth and seems to work for me. The last thing you want is heat exhaustion, which is no fun at all. . . .
Ugly John