They aren't just towers in the dark. These things have all sorts of sensors on them. I doubt even a submarine is going to stealthily destroy these.

The power grid has way, way more weaknesses that would be way more impactful dont require a submarine to execute. Capitalism has guaranteed we have basically zero spare parts for large transformers. A single person can easily destroy these and do more damage in a war time scenario.

If I sabotaged your offshore power, I'd say that I didn't and have my navy produce a report that concluded that whales did it.