3 liter Osprey hydration in the Osprey pack.

Backing that is a 4 liter Platypus Gravity Works filtration system (added the charcoal canister but frankly don't think I need it in East TN) that stays rolled up in the pack. If I'm going to hit a dry stretch I can run a few liters from the platypus to fill my hydration, then run another batch to fill it. That gives me 7 liters of fresh water that's better than anything I have at home. If it looks like I might need more than I can simply fill the extra 4 liter 'dirty' bag and that gives me 11 liters of water to make it to another stop. For what it's worth, I've been out a few times with 3-4 people and one (1) fill of the 4-liter bag has seen us through 3 days at a stretch of cooking, cleaning, etc. I can't imagine ever needing 11 liters short of ferrying a boy scout troop across the desert.

Side bonus: Any and every little stream, creek crossing, or lake shore is a source of immediately drinkable water.