If you notice, water bottles now have ridges and stuff in them instead of being smooth cylinders. The reason is that there were several cases where a bottle of water in a car at the right angle and position focused the suns rays and started a fire. With the ridges and other texturing, it's far less likely. But a smooth cylinder of water CAN be an improvised lens.
As I said, the trick is to try out a lot of unconventional things so when you need it, you know you can start a fire.