Even if there is only one set of pipes under the street, the owner of those pipes is constrained from acting in a predatory way if the market is free. They are disciplined by potential competition (if prices too high or lets the water quality drop, it suddenly becomes financially viable for a competitor to dig a rival pipe, or set up localized system, or offering water trucking to your site).

Under a government-granted monopoly, that threat is legally banned (except for maybe water trucking)

You are correct, but only at unrealistic levels of bad. A potential competitor risks that they lower the prices after the have dug in the pipes and that is enough that the competitor is bankrupt. Even if water quality is the issue, it is easier and cheaper for the incumbent to fix the issue than for you to dig in new pipes. Only in the case where the incumbent has worn out pipes that all need to be replaced at once (unlikely - pipes last a long time and we have had good success relining pipes in many cases: leaving only a minority that need expensive replacement at any one time).

Because the bad has to be unrealistically bad before someone can compete nobody sane will.

Government granted monopolies are granted under oversight and regulation that would be extreme in any other scenario. Many of them don't even get to set prices. No discipline or market forces needed. The regulators just regulate predatory action.

In your example no one is going to dig a rival pipe because you can't just go around installing pipes under other people's roads and land. The permission to do that is granted to a single entity by the government, further reinforcing the natural monopoly. There is no situation where it is economically feasible to lay more pipe when you have no right to do that.