I don't really get that dehydrated because I mostly play in indoor airsoft arenas. But when I do go into outdoor ones, its hot like the devil's piss (lol) But I carry a hydration pack full of Kool-aid (No lie) and I carry a few sandwiches. That's pretty much it.
I'm 87 Pounds, Really skinny, so I don't have to carry much weight on me. Plus, I run a lightweight recon class.
I never get headaches during airsoft games. The only exception I've had was when a loud airsoft grenade explosion was right next to the wall I was leaning against. But yeah that's pretty much it.