The last boat used for this stunt was Chinese, this one is registered in the Cook Islands. This is already happening.
There is no way out of the Baltic Sea without crossing the territorial sea of either Sweden or Denmark. Countries have full jurisdiction in those, cutting cables on purpose is at least a criminal act, if not terrorism. There's no problem handling this, even if you're fully playing by the books.
Russia would then need to switch to ships running only to Kaliningrad, which makes it even more obvious that it's an act of war.
There is no way out of the Baltic Sea without crossing the territorial sea of either Sweden or Denmark. Countries have full jurisdiction in those, cutting cables on purpose is at least a criminal act, if not terrorism. There's no problem handling this, even if you're fully playing by the books.
Russia would then need to switch to ships running only to Kaliningrad, which makes it even more obvious that it's an act of war.