One of the major issues I think people complain about with publicly owned services is that they create a monopoly, which leads to inefficient outcomes. So if a bunch of foreign national companies are all competing in our national market, that means the monopoly effect isn't present in our national market.

If a foreign national water service in another country is a monopoly, then it will have a captive audience to generate revenue from, which could provide them with the resources to compete in our market. This doesn't necessarily mean that they would be competitive enough overall to compete with private companies generally.

This is a bit off the cuff, and I have no economic education, so everything I've said could be tosh though.