I've been stockpiling Subway bags for a while, so afraid they will change them and they'll be gone for good. They are very strong for their weight, and compress down to nothing.
They are good for many tasks, from vb socks to keeping fleece gloves dry in the rain. I would only use them to cart water in an emergency...
In addition to bags from boxed wine and coffee, I find these to be light and strong, and trust them inside my pack: < http://www.amazon.com/gp/aw/d/B001E5E0GA>. And as with all reused containers, bring an extra cap, as that will most likely fail first.