So does nationalization cost anything? It would happen by issuing debt to acquire an asset - a company.
If transactions costs were small, and the market price knowable, the transaction cd be reversed at any time leaving the govt at square one.
Might be a 'cost' to extent that the net revenue stream acquired injects risk. In same way that private sector QE transfers risk to public.
At times - hence private sector QE - that transfer, though costly, might be beneficial, if market's ability to bear it is impaired.
In case of strategic companies like water, risk transfer might be smaller than it appears, if public sector implicitly bears this anyway.
Next q is what the govt does with company strategy, and what it does with the revenues.
Stark eg: Venezuelan oil. Sold at subsidised or zero prices. Staffing used as patronage, etc. Not a serious comparison; to make a point
If public ownership used to change company strategy in a way that impairs the revenue stream; this is a 'cost'.
That includes, eg, making eg water company 'profits' more likely to be diverted away into govt transfers.
Equally, nationalisation might in principle make regulation simpler, eliminating rents, improving efficiency.
Aside from effect on narrow revenue stream, this could improve other govt revenues from elsewhere in the tax base.
Borrowing money to buy a house not a good comparison. Since here I am borrowing to consume the housing services.
Better comparison is borrow to buy and then let out. But only slightly better. Here the risks for lender are large.
ie large relative to my other revenue streams that would be there to service/repay the debt.
Also, there's the issue of credit / management histories. UK credit history good, apart from quite a lot of 1950+ inflation/fin repression