Water! Glorious Water! This is the biggest item you will need to plan around. At minimum you should be trekking 1 gallon of water per adult in your pack. With that amount you would feasibly have some left over for other uses (cleaning, washing, cooking etc). You may want to include more if you have room considering you’re more than likely to be in a high stress/physically demanding scenario if you’re breaking out your pack.
Containers: Rather than grabbing a gallon container and sticking it in your pack, consider using smaller containers such as bottled water or water packets (like Capri Suns, but instead of juice, they contain….you guessed it, water!). That way, you’re safe in such situations as your gallon container breaking, or getting contaminated. If that happens, your whole supply is gone. With smaller containers, you have a bit more diversity as to storage and use.
Other Beverages: If you have room, you might consider including a few juices or sports drinks. The latter are designed to help athletes rehydrate and replenish electrolytes during training/competition. They will do the same for you, and give you some extra calories because (again) you are more than likely to be in a high stress/physically demanding scenario. But make sure these are extra, and not a supplementation for your water. Water first, then other drinks.
My best suggestion regarding hydration is….carry your required amount of water and just include some sports drink powder packets. That way you can pick and choose if you want to use them or not.
The “No-No” Beverages: By no means should any pack include a soft drink/soda/energy drinks/alcohol. These are useless in an emergency scenario, and this is coming from the world’s #1 Diet Coke fan. They will dehydrate you, take up room in your pack, offer no nutritional value, and some will impede your judgment and/or response time. You don’t want to be out of sorts during a disaster situation.
Hydrating your Crew: Think about the water needs for the rest of the people you are making packs for. Pets need water for drinking. Babies need water for formula. Some food may need water in order to prepare it correctly. Overestimating on your water needs won’t be a bad thing.
Just in Case: Be sure to include some water purification tablets (these are the most convenient in size, function and price in my humble opinion) in case you run out of water and need to scavenge for some.