The ‘clean’ alcohol instead of ‘dirty’ water story is a myth. There’s plenty on the internet about it, it’s been debunked many times, but here’s the first link I could find for your reference — http://www.medievalists.net/2014/07/people-drink-water-middle-ages/

If nothing else — although there are lots of other reasons it’s a myth — ask a simple question. If you can cleanse water by boiling it to make alcohol, why not just boil it to drink the water?