Yesyesyes, clean water, that's a close third of course. Living in a place where tap water has over 10 times the concentration of harmful bacteria than the WHO-recommended maximum for public swimming pools plus a *cough*healthy pinch of salt I know a thing or two about clean water.
Apparently you don't know how to make clean water :)
Preparation is key to survival, gain the knowledge before it is lost :)
Sustainable Decentralized Water Treatment for Rural and Developing Communities Using Gasifier Biochar, Production of Ceramic Water Filters, Biosand Filter Manual, Official Site for the BioSand Water Filter, Solar stills, WAPI (Water Pasteurization Indicator) / SODIS, Shapla & Surokka arsenic filter