You are evidently unaware of UNCLOS and the adoption of many of its provisions into customary international law
(a) the ship is engaged in piracy; (b) the ship is engaged in the slave trade; (c) the ship is engaged in unauthorized broadcasting and the flag State of the warship has jurisdiction under article 109; (d) the ship is without nationality; or (e) though flying a foreign flag or refusing to show its flag, the ship is, in reality, of the same nationality as the warship
Which one would you like to use to board and/or force the ship to depart against Russian cable cutting ships?
But the answer to your question is a. Referring to UNCLOS 101(a)(ii) the cables are "property in a place outside the jurisdiction of any State".
High seas (which is what that list applies to) is not the EEZ. I don't think anybody could legally argue thar a country wouldn't have the right to board (or fire at, if it didn't comply) a foreign ship from it's coast 24 nautical miles if it suspected it was doing something illegal. Whether that right extends to the entire EEZ isn't exactly clear.
However there are no "high seas" areas in the Baltic so all of the listed items are irrelevant.
Of course the Baltic is very shallow so if the reactor started leaking it might be a bit more problematic than if a nuclear ship/sub was sunk in the middle of the ocean.
If only there was such thing.