This smells wrong to me. You already have people working at the water treatment plant. You don’t need it connected to the internet?

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.

There is lots of infrastructure related to water that doesn't have someone physically there 24/7 as it would not be feasible to do so as 99.9% of the time there is nothing to do. So you need some sort of remote alarm system that can be monitored.

at least disconnect the poison valve