The people withholding the water in this scenario are the ones victimizing the ones without. That's where the state monopoly on violence has to come in as a corrective mechanism.
And it's unjust to assume humanity wouldn't help unless forced to.
By calling upon sacrifices, the first target would be engineers, plumbers, and utility workers. Forcing the people who actually produce and deliver clean water isn't justice.
Punishing and preventing it is the core purpose of the state.