I wonder what would be the bare minimum required to stage an operation like this, if one were willing to take big risks. Like, combat divers operating from an inconspicuous sailing yacht or the like? Sail into position, go down (80 meters should be doable with Trimix), place explosives with timed fuses (set to a few weeks?), go back up, throw the equipment overboard later, sail into Kiel with no one the wiser? Not that such a scenario is very likely, but there have been a lot of crazy stunts in this and other wars.

I'm certain that there will be investigations from NATO navies soon, and we will have some explanation. Whether or not that's actually what happened or not will depend on who did it and who is most beneficial to pin it on, but I'm sure there will be pictures that are illustrative.

That's like week 8 of BUDS (basic underwater demolition system) in SeAL training.

They have combat divers (one of the special forces units is even the 1st Underwater demolitions Unit, apparently) and the average depth of the Baltic is 50m. So my guess would be: yes they have the capability in principle.

Ukraine is a proxy in a proxy war. Ukraine would have no capability to do this, but USA would.