I just drink tap water.
I was sick while travelling in China, and one time I woke up painfully thirsty. I knew that you couldn't drink tap water in China, so I desperately went through all my bags and the entire hotel room looking for a trace of drinkable water (I had plenty of used bottles, but they were all empty). Eventually, I found a kettle behind some bed sheets in the cupboard. I boiled some water, poured it straight into an empty bottle, ran it under a tap for about fifteen seconds before drinking it all in one gulp. I burned my throat, but at that point I didn't really care.
Afterwards, I boiled more water in the kettle. I then poured the boiling water into the remaining plastic drink bottles I had. Needless to say, plastic and boiling water don't mix well...