You might want to check out the Clear Zip Lock Poly Storage Bags at Safety Central. I have ordered various sizes of these. They are excellent 4mil thick versions, which should stand up to more abuse than your average grocery store brands. When filled full with water and turned upside down, they do leak little drops of water. When filled full with water and laid on their sides, they leak little drops somewhat more slowly. So no, they are not "watertight", at least not when full, not unless stored upright and not if squeezed harshly or otherwise manhandled. But then again, my mil-spec 5qt water bag and mil-spec 2qt soft canteen each did the same thing. I had to make sure I kept them hanging or sitting upright if they were full or nearly so, or they would slowly leak, at least for a while until they had less water in them. So, I consider these particular zip lock bags to be as "watertight" as mil-spec canteens often are, which is to say, good enough for me in an emergency.<br><br>(For what it's worth, I ultimately took my canteens to a local hardware store and fitted their caps with appropriately sized O-rings. That solved the leaking problem. I make sure to keep an extra O-ring for each, along with my Potable Aqua, in the little pockets provided in the canteen covers for the iodine tablets, in case I lose the O-rings already in the caps.)<br><br>The 6"x8" bags (which are actually more like 6"x9") hold 40oz when completely full (roughly 1180ml or so), but will leak somewhat less and are much more managable if fillled with only 30oz or so (900ml). The bigger bags hold ridiculous amounts of water... far more than I would consider realistically portable.<br><br>Oh, and despite the images used to illustrate the product, these bags are, indeed, clear.