There is simply no way to know what is in any water tank on any boat or the condition of the tank wall and piping. We alway have bottled water on any boat we are on. Even the best city water has a shelf life where the chlorine or whatever is used to inhibit growth reaches the end of of it usefulness. Every boat or water system sits idle at some point. Why risk anyone getting sick in the small confined space of a charter boat. Having the bottled water aboard also serves as a valuable backup if you water pump fails or somehow find your tanks empty. Having bottled water delivered to the boat is very easy these days. We also try to store away one gallon jug of water per person somewhere. If we ever find ourselves unable to get water from the taps. "Here is your gallon until we work this out".