I never was able to wrap my head around this argument. If something is both (a) not commercially viable and (b) necessary, then it should be funded by taxed and operate (gasp!) at a loss.
Capitalist economics work really well for certain problems, and abysmally for others. Public water works falls into the latter category, as evidenced by Flint.
I happen to disagree with the argument I just provided, but for reasons different than the quality of service a free market would provide.
That's not a realistic scenario however as that implies that each company would set up its own plumbing infrastructure, water sources, etc. In reality, the infrastructure is shared. So I see no reason why the choice wouldn't target be between "company A who has lead-laden water, company B who does some extra purification and charges for it heavily and company C who serves the exact same water as A but has better advertisements."
What would be the incentive to actually fix the infrastructure if it benefited your competition just as much as you?