Water and electrolytes of some sort. Gatorade is not amazing but its better than nothing. There are also electrolyte tablets out there that you can mix with your water.
What your packing also depends on the enviroment. If your in the woods an extra set of everything helps because if it rains your going to get muddy.
Urban you wont need two sets of boots. even if it rains.
As for batteries it depends on how much you shoot. I've had my battery last for a 24 hour game with no problems. The more you shoot the more the battery drains.
Eye pro you ll want to break in before hand you need a comfortable pair so your not distracted. You might also have to sleep with them on depending on the situation.
A vest helps but is not needed but recommended. You can easily carry all your mags on a belt. But if you have or want to carry more gear it is advisable to bring a vest. A camelpak is also helpful.
killrags are greeat for avoiding getting shot after you ve been hit. Same for a red glowstick at night time. Other glow sticks can be useful such as by throwing one to mark where an enemy is.
A matching uniform is usually required to be part of a mil sim it is a mil sim after all. Helmets are up to you. I wear a baseball cap usually. But if you want extra protection get a helmet.
Food bring a few things that are quick and easy to make. Cans of soup, granola bars , ect.