Water is the essential nutrient. Life cannot sustain without it. If your only option is dehydration or tap water then obviously drink the tap water! If you have the opportunity to choose the healthiest choice, then choose something better.
There is considerable concern about the risk of tap water being contaminated by various substances and organisms.
Water from dams, rivers, bore, or tanks may be contaminated by fuel emissions, industrial waste, human waste, chemicals, sewage, agricultural waste (including pesticides, fertilisers and animal waste), chemical spills, micro-organisms or drug residues.
Water treatment may be insufficient to clean the water and remove dangerous chemicals or organisms, or may inadequately monitor the levels of certain substances in water.
Treatment plants add chemicals to water that are designed to improve its quality and safety, however there are questions regarding the safety of some of these substances. This usually involves the addition of chemicals to the water that destroy micro-organisms, or improve its look, smell or taste. Common added chemicals include Chlorine and Chloramine, Aluminium and Fluoride.
The pipes that water travels through on the way to your house or once in your house may be contaminated, made of dangerous metals, or in poor condition. A common source of unavoidable copper comes from copper piping.
So now what do I drink? I am not saying you have to drink bottled water as truthfully bottled water has question marks as well. The best water to drink is spring water as can be used as a therapeutic agent due to its mineral content. Alternatively, invest in quality water purification or filtration methods to protect you and your family from heavy metals, chemicals, toxins and other nasties.