Water is critical to all life and is your first priority. You will need at least 3 liters of water in your bag to last you 72 hours. You will use water for hydration as well as hygiene. I suggest carrying at least 2 water containers. I carry two Nalgene wide mouth's for water containers. I like the wide mouth because its easier to collect water with. One of my bottles is Lexan and should never break, the other is Nalgene's soft bottle that I can bend, smush, and manipulate it to gather water if I need to. I also carry Emergency drinking water packets as extra water. These packets can be carried in pockets or divided up.
You will also need to include a couple of water treatment methods in your bag. I carry a small Frontier emergency water filter and Iodine tablets. These methods would provide weeks of clean water. Remember this is a Bug Out Bag and is only designed to get you through the first 72 hours. These methods will help you gather more water if it goes past the 72 hours. You can also boil water but I wouldn't rely on that method, the situation may dictate otherwise.