Only a few folks have a true motive and capability which includes being able to get there and out undetected.
I seriously doubt the Ukrainians did it, at least not without our direct help.
They do appear to have a SEAL / UDT type of capability (1st Underwater Demolitions Unit), https://www.kyivpost.com/post/17126.
Getting to the Baltic Sea and out undetected, that would be a bit of a stretch. That is completely out of their AOR and they do not have a presence there like we do.
This pipeline is something with ramifications across all of Europe, for NATO potentially, and to execute this target the decision authority quite frankly was likely the POTUS.
Depending on the target and the consequences of going after it, you need higher approval and something like going after this pipeline most certainly had Biden giving the OK.
I would bet my money on Seymour Hersh’s version. The Germans began an investigation and pointed the fingers fairly quickly at the Ukrainians. I suspect this is a Strawman and Red Herring. If it’s the US, that would cause problems. Better to blame someone else that is believable, but where the consequences aren’t so severe.
I'm sure the Greens are OK with this... until they start freezing in their Yurts.