Here is an image of a bag I found in my local auto parts store.
It's called, simply, an "Emergency Water Bag", and it's made by "Justin Case".
It's sold for drivers to have something to carry if they need to haul water to fill an overheated or leaky radiator in an emergency. Filled to it's clearly marked fill line, it carries 165oz (>4800ml) of water comfortably and sturdily, so it's got all the capacity and more that anyone would need. I don't know the thickness of the plastic, but if feels strong and the seams are well made and tough, even in the corners. It's certainly nowhere near thick enough to be a primary or permanent container, but, for short term, I felt it should do just fine.
I kept it sitting around full for about two weeks and it never leaked. I dropped it into my bathtub from chest high and it held. Unlike other bags this large, this one inspired confidence that it could handle the weight of that much water. The only problem issue was the plastic "Kwik Lok" tab. When held upside down, the weight of the water slowwllyy caused the twisted plastic to untwist, despite the lock. I solved the problem simply by tying a knot in the plastic top (a process which was greatly facilitated by the triangular shape of the bag) and clipping the tab on above it.
I thought it was an excellent option for an emergency water container for larger pocket kits, since it folds up as small as any similarly sized (large) bag, but was tougher than many and the triangular shape made it much easier to carry. It even stood up on its own more easily than squarish or rectangular bags I've tried.
When I searched, I found many many links to these all over the web. I had never heard of Justin Case, but, apparently, they sell lots of travelers' emergency kits online through many different sites, just in case (ew, sorry) anyone was interested...