As a 'Zone Man,' I am well aware of the dangers of dehydration and heat stroke especially when out for an infernal hike. Although a U.S. gallon of water weighs 8 1/3 lbs, those are pounds I don't leave home without. Some will be surprised to learn that even with water there can be too much of a good thing.
Thales take note!
The danger is increased if you drink pure water. Since my reverse osmosis water purifier delivers water that is around 95% pure, I add electrolyte replacements such as Gookinaid to my water or else bring along salty snacks.
In fact, the sort of greasy, salty, sugary crud that you shouldn't eat at home makes for good trail food.