Under the sea who know for sure who made what? Even an ally can do such sabotage just to generate a military response against a common enemy he want to aggress but can't do without allies and those allies are not much interested if not forced or even a simple self-inflicted damage to justify a war the public do not accept but some in the government/élites want... Did you remember just Britannia to cite something from history or the outcome of the attacks of Charlie Hebdo, Bataclan and Nice have generated?

Such attacks can even disable half a country grid just due to the big drop in frequency of the whole grid, spark enormous outrage and can also be both really damaging and really quick to fix. Life it's complicated, where no one can easy see anything can happen, design things to avoid potential attack surface is a good way to protect themselves.

We do not normally have strokes, but we put defibrillators everywhere for a reason, we do not have much car accidents but we design cars to make us survive important ones etc. A critical infra like an energy grid... Oh, sure we in the west are equally dependent on foreign supply chains for oil, uranium etc but we stockpile them a bit, months for oil, an year or two for uranium, it's not the same of electricity in a grid...

Just see how many issues are there due to some supply chain issues in south China or the "Ever given" crisis in Suez. We are already hyper-vulnerable planning even bigger vulnerabilities for the future it does not sound that smart to me.