A water reservoir like the Platypus Big Zip hydration system, that can be rolled up when not in use, may be the best solution for one of the harder problems of mobile urban preparedness: having enough water. You should have 3L (100oz) of water per person per day, and the ideal go bag must be able to carry you for more than a few hours.

So, have all the full and sealed plastic water bottles you can carry, but also bring one or two empty 2L portable hydration systems with you, filling them when you have the chance and safely accommodating it in your go bag – if you don't have to flee in a hurry, it's highly probable that you'll be near some potable water source anywhere urban you might be when the things start going wrong. And if you have to run, use the water bottles until you can find a safe water source to fill the Platypus.