2x 1 litre field canteens (one on hipbelt of the rucksack and another in backpack) for drinking and one 1,5-2 litre soda bottle for cooking, dish washing and as extra drinking water if I run out of the water in those two canteens. That's one days worth of water and with that I can sustain myself without having access to wells/streams/lakes for 24 hours. I keep 3-4 litres per person per day as a basic rule when calculating the need for water. That's the same we had as thumb rule during our conscription as well.
During winters I only tend to carry two field canteens and keep them inside wool socks to prevent them from freezing, because then I can resupply from snow.>>2218945
In army exercises we carried up to 5x of 2 litre bottles (plus two one litre canteens) in our rucksacks because in those said exercises our operational areas didn't have water availeable and we were dropped in the middle of "combat zone" in which we had to evade from both parties meanwhile we were moving deeper in to the hot area, so we weren't given resupplies untill many days in to the exercise. We were rrsupplied with field caches that had mostly some freeze dried food and one 10 litre cube of water for a whole squad. Fucking loved recon exercises.>>2226373
Shouldn't just about any PET bottle be suitable for that?