I use "RPavlis Water" in my espresso machine. Deionised water, plus 100ppm potassium bicarbonate (~0.085g KHCO3/L H2O). It's nonscaling, provides enough mineral content for fill probes etc to work, and keeps acidity to a bearable level. The water that comes out of my tap is scarily close to RO here in the Boston burbs.
If you want more information on water chemistry, Espresso Aficionados has you covered.
If you want more information on water chemistry, Espresso Aficionados has you covered.