1 quart Ziploc freezer bags work pretty well if you don't overfill them. You can burst one if you try really hard, but they'll survive normal drops to the ground, carrying in a backpack if not squashed against something sharp or pointy, etc. I'd avoid using them as a primary water container unless absolutely necessary, but I've used them to carry extra water in an external backpack pocket (nothing else in the pocket to puncture the bag or get drenched if the bag breaks) and haven't had any trouble yet.