When boiling water with hot rocks, if you're using a plastic bag set into a hole in the ground, both the ground on the outside of and the water inside of the plastic bag tend to become insulators against the heat of the rocks. So generally, you won't have to worry about burning through the plastic unless you've got too little water inside the bag, the hole is too shallow outside, or the bag is making direct contact with the hot rocks with no insulation between them.

Still, I've seen people line the bottoms of bags with a few cold stones, bark, and even a few sticks when using this method to boil water, so I could have been misled regarding the insulation.

Has anyone else heard this?

Perhaps I've just been lucky! <img src="/images/graemlins/laugh.gif" alt="" />

You can also hold the hot rocks in the water using sticks as "tongs".