Water companies, whether privatised or not, will have the same income streams
I think your example of Tideway is the exception rather than the rule. And in that scenario the government isn't actually borrowing the money, it's backing it. Governments can borrow at lower rates but will they borrow, or rather, will they enact the legislation in the first place that means they will have to borrow. (2022 estimate for delivering the storm overflows bit of the Environment Act is £178bn, so add some inflation onto that. Source: https://assets.publishing.service.gov.uk/media/624460a18fa8f527744f0655/storm-overflows-evidence-project-march-2022-addendum.pdf).
I'm not really arguing for or against privatisation, but I wish that people wouldn't talk about nationalisation like it's some silver bullet that will fix everything they don't like about the water environment. I'd love people to have clear ideas of what their aspirations for watercourses are, to know what it would take to get there, and to be cognisant of the challenges and implications (delivering the Environment Act is going to release A LOT of carbon. A LOT.) of getting there. I think the press have let the public down badly on this front. Just my opinion.
Like you say, that's your opinion. Public utilities should all be state owned to remove any semblance of profiteering first of all. You also state that the money will come from the public purse but that's already what's happening anyways as the water companies operate from profit they make from us, the public. The only difference would be that instead of shareholders pocketing 70m every year that extra money could go towards offsetting or capturing that extra carbon from delivering the Environment Act.
Also if the press were doing that piss poor of a job communicating issues then surely you guys as private entities could sue newspapers for libel. Except the issue is that you can't because no lies have been told, we can see that from the fact that you're getting fined millions.