Pure water will work for a couple of percentage points but above that will not work properly because atomisers expect a certain range of viscosity or they won't wick properly. It's generally a mix of propylene glycol, glycerine, and water. More glycerine means more clouds, natural sweetness, and annoying hygroscopy (i.e. you'll get a dry mouth), while PG is an aroma carrier, less sweet, quite a bit less hygroscopic. It's also the standard solvent for nicotine and aroma, not just vape aromas most food aromas are PG-based, too. Water is there to make the liquid less viscous and/or reduce hygroscopy of the overall mixture.