Hello there!
There are two answers to your question, and that would be good water supply and bad water supply.
Lets talk about good water supply:
Good water supply is very beneficial the people's health, including the public's health. Good water supply helps the body digestive system stay healthy. Good water supply also supports the prevention of dehydration in the body that no one wants to go through. If you dehydrate, you'll pass out, and water takes away that factor by hydrating you.
Let's talk about bad water supply:
Bad water supply is very bad for people's health and the public's health. Bad water supply would be very dirty, in which people would be drinking dirty water that would not help them, but to make their body dirty and unhealthy. I could lead to many sicknesses like diarrhea, vomiting, headaches, etc. This would be the same as drinking water that has dirt in it. When you drink water that has been badly supplied, you're pretty much drinking water with a lot of bad bacteria in it, in which should not go into the body. This is a big problem amongst different countries and nations around the world, due to the fact that some countries don't have access to good water supply.