I know the weight weenies will love this one but:

I generally carry four 33 oz. Zefal Magnum bottles. I keep two in the bottle cages and two spares stashed in the rear pockets of my pans. I buy a gallon of drinking water at a store and fill all four bottles, usually in the morning. That's enough water to last all day and sometimes enough for coffee and cooking. A regular gallon of water only costs a buck or so, so I spend a dollar or less a day for water.

Sure, a gallon of water costs 8 pounds or so. But as the day progresses my cache of water gets lower and lighter.