What follows is entirely my own opinion, based exclusively on open-source.
This benefits from the questions of classic homicide investigation. The suspect must have motive and means. It is likely that these actions were committed by someone on the Side of Good, for Good Purpose:
- To obviate backsliding by Germany, which has been notably slow in providing advanced weaponry to Ukraine. This shuts down the wistful hope for a return to the past, which could have fractured the Alliance.
- Of equal import, it demonstrates to the Russians that their premise of weak European solidarity, that Europe could be bought, will continue to be wrong.
- The calls for investigation are quite tongue-in-cheek.
As for the means, the Baltic is quite shallow at the locations. A submarine with a diver lockout chamber is not required. Neither is fancy pipeline repair gear. The depth of 187 feet is reachable by competent scuba divers breathing commercial trimix gas. The dive boat need have no visible adaptations that distinguish it from a fishing vessel.
As to whodunit, I leave it to your imagination. There must be a lot of smiling around office water coolers. As George H.W. Bush famously once said,