I find usually carrying 2 bottles of water directly on you is enough. You don't down a bottle of water when you're thirsty. Just drink enough to prevent dry-mouth. Some people carry a backpack with essentials in it. Personally, I have a vest with a radio holster and an auxiliary pouch. They each hold one water bottle for me. If you have more mag pouches then what you use, shove a bottle in there instead.
Things like BBs, a speedloader, screwdriver kit, etc, I'd suggest buying an ammo box for. Carry the ammo box to your base and leave it there. Don't forget it, obviously, when returning to the clubhouse or whatever. Usually other people will leave their accessory gear at your team's spawn as well, and it'll get picked up if you forget anyway. The only thing I'd suggest carrying on you aside from things like small food and beverage is the speed loader. Sometimes when you're in the middle of a fire-fight, I find that speedloader gives you about 1.5-4 mags (depending on your speedloader size), and saves your ass. Although, it rattles. can be bad :\
Primary: Custom WE M16A3
Secondary: KWA MP9
Side: Custom WE M1911A1