The overhead of having shareholders in this case is minimal. The profits they make are small, but having a goal of making profits does create discipline in resource usage.

The water system is like the electricity system. It's perfectly possible to have inflows and outflows be fully private, as long as the government keeps its hands off the pricing. The network itself can also be run privately, as both supplier and buyers want supplies to flow. The trick is to ensure there are numerous different companies with the expertise to maintain pipework and then allow local communities to quickly change to different contractors.

This is why the state owned company structure exists. The board and management are correctly incentivized and the profit can be reinvested if necessary. No parasites.

You need a counterbalance to efficient resource use, usually competition ensures they don't skimp, that doesn't work with natural monopolies.

Kind of shows how toxic things have become in our culture when people need to be bribed with profits to provide the basics necessary for a society to function, instead of just being incentivized by wanting a functioning society.