Water is the underlying quality that makes a beer what it is. Guinness would not be what it is without the liffey water. Pilsner urquell would not be what it is without the soft pilsen water.
Water is vital to beer.
Nowadays we understand water chemistry so it's less important. Humans can now recreate water as we see fit. However, can the monks do that, do they have chemists/scientists to do that for them? I'm not sure they do.