Seymour Hersh published a detailed piece outlining his case that it was carried out by the US military with assistance from Norway (who have expertise in that part of the seabed), using the cover of a NATO naval exercise as partial cover:

https://seymourhersh.substack.com/p/how-america-took-out-the...

I find it more credible that these were the actors involved. Biden wanted to end a future link between Germany and Russia. In Germany there is significant segment of the population that is distrustful of the US (even before the recent Trump shenanigans) and the potential stronger infrastructure ties to Russia. I find it more credible than a ragtag group of Ukrainians did it personally.

Although the truth may never be known.

Edit: for clarity