OK, here's what I found out.
With thin sandwich bags, forget it. They started blowing holes just about right away. Also, if they weren't full, the heat goes around the water, and melts the plastic above the water.
With a full quart freezer bag, I started to get somewhere- but where there were little wrinkles on the bottom of the bag, it started blowing pinholes.
So, I tried a quart bag on it's side, where it's smooth. Really tricky to hold that way, without getting burned. It never sprang a leak, but the zipper part started melting with the heat going around the bag, and I was losing the hair on my hands, so I quit.
Conclusion: it might be possible still, if you can manage to hold the bag in such a way that only smooth plastic with water on the other side is in contact with the heat. But damned if I can figure out how to do it in the comfort of my kitchen with a reliable gas stove. In a survival situation, no way.
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Benton