Most water treatment works are unmanned most of the time. You don't need 24/7 staffing or even close.

But more to the point, a modern water network has a huge number of nodes. If you can't centrally aggregate and control in a control room the costs and complexity explode, probably also the error rate.

Even if you demand a full air gap, the solution here can't be to get rid of computers or networks. They are much, much too valuable. Luckily industrial control is full of very low hanging fruits.