It's important to have clean water flowing through your body to carry off waste and toxins. You wouldn't wash several loads of clothing in the same load of water. Toxins accumulate and store within the body when they can't be eliminated.
Water is the universal solvent.
It is necessary for life and good health. The dry mouth is the last outward sign of extreme dehydration. Damage is occuring to the body before this symptom appears. Because of a gradually failing thirst sensation, the body becomes chronically and increasingly dehydrated. This is a good place to start.