The Reinheitsgebot just says water. It says nothing at all about the water quality. Today German brewers trest their water by softening, RO filtration, degassing, and so on. CaCl2 and Gypsum are allowed as water addition.
Ayinger has a 603 meter deep well that has water that tastes wet, you sample it on their tour. It brings up water that comes from high in the Alps.