Cooped Up? Get Uncooped.

While this does seem an elegant and effective solution, I, personally, would use other methods to try and purify/sterilise the water if I was planning on drinking it (such as using a solar still).

Having done some quick research on the net, I have found sources which suggest that water must be boiled for 10 minutes (and that's boiling, not just bubbling) - http://foodsafety.ifas.ufl.edu/HTML/nc330.htm - and others which point out that you may need to boil for 5 minutes as a minimum (due to altitude/air pressure having an effect on the boiling temperature of water) - http://www.easyfunschool.com/article1541.html. I am not sure whether there would be sufficient volume of water in the plastic bottle to sustain a boil of that duration.

As I said, I wouldn't recommend this method, even in a survival situation, unless it was your absolute last resort.