well i think if your filling water youd wait till you know you are safe like in your house so fill time would be nothing. youd maybe have to change the size of it make it smaller but still good enough size people wouldnt just dump it and carry a bottle of water
I like the idea, if you're going to get supplies, the backpack has the water pack and a straw which you drink from, allowing for less stops and messing about with your bag, whether they can implement that into the game, god knows...
I personally own a Camel Bak daypack for highschool that had its hydration sac/water tube removed by its previous owner and a much smaller camel bak that still has its tube.
The rubber tube is pretty hard to break in normal usage and the mouthpiece is better made than that of any water bottle I've ever owned on everything camel bak manufactures. I can't name any other companies by brand that incorporate hydration with transportation, but it is a feature I'd look for if I was in the setting. They're especially great psychologically; you start the day really heavy, you end the day however many litres of liquid lighter. The only drawback is the slosh-slosh when it's between full and empty.