It does look like the Ukrainians are in the frame for this.
There's a US Journalist who outlines how it was planned -
link here, he was interviewed on UnHerd (
link here), his account to me is somewhat convincing.
As a keen diver I'd add a couple of observations.
He maintains this was a group of military (bomb makers etc) who used civilian divers. That makes perfect sense to me. Many amateur divers are more experienced and competent than their military equivalents. I know of people who are regularly diving wrecks on rebreathers in the 80m range. An 80m dive isn't
that unusual, huge pub bragging rights, but being regularly carried out in the somewhat more challenging (diving wise) UK. They're also used to working at depth (pulling up portholes, grabbing lobsters etc). So placing charges at 80m would be perfectly feasible. But...
What does puzzle me though is that a yacht was allowed to stooge around repeatedly over such a sensitive site. (They apparently dropped off charges at a couple of locations). Finding stuff underwater is hard. Landing on it is even harder. It often takes a few goes and can be very, very hit or miss. Lots of runs over the target and several attempts to shot / grapple it. Divers can't swim down in such a situation - they would have had to descend either the anchor line (so the yacht would have to anchor into the pipeline) or a separate shot line. They wouldn't have had to contend with our (UK) tides so could probably dive whenever they liked but Its the most surveilled sea in the World, and surely even a yacht stooging around looking for a 1 > 2m high cable on an echo sounder would have rung alarm bells in Sweden, Denmark or the US. Either a blind eye was turned or well, there's no surveillance after all.
I personally suspect that as the poem goes 'some people looked at the wall while the gentlemen went by'...