... an insulated bottle that you could boil in ...
Yeah, it does sound funny at first glance.
However, it wouldn't be insulated when I got through with it. I'd be removing the bottom of the outer shell, so I could hang it over a small-stick fire.
Think of it as a vertical pot with a shroud to concentrate the heat and increase efficiency.
Once it cooled off, I could put the stopper back in to transport the now-drinkable water.
If I really wanted to, I could stuff some foam between the inner and outer walls, and voila, it's insulated (somewhat).
Wacky idea, I know. But I see a lot of people with steel thermoses, and I wonder how I could convert them into something useful in a tight situation.