What difference would you expect from nationalisation over the current system where water company spending is constrained by the regulator and government isn't concerned by debt as it's not on their books? Apparently Ofwat have prioritised low bills over investment over the last decade or so. The companies are encouraged to invest by the structure of their monopolies as more investment means a greater ability to make profits.A monopoly I can think of in this country are our water companies, which are a geographical monopoly, so you have no choice but to go with whoever the company is in your local area. If we could only nationalise one utility industry in this country, I'd be going for this one, we've seen evidence of how ineffective private water companies are through billions of litres lost in leaks that are not profitable to fix, and in all the untreated sewage being dumped in rivers and bathing waters.
I can't find the source I saw before, but UK water leakage rates were about average. I suspect based on stories such as https://www.waternewseurope.com/tourists-in-europe-are-swimming-in-raw-sewage-despite-blue-flags/ that our sewage discharges are probably fairly average too, we just don't see the reporting of the issues in other countries.