If you want to do it undetected you need to do it completely underwater, including ingress and egress, and that absolutely limits the potential actors.
Also, I understand the depth of the pipeline is 80-100 meters so you need divers experienced with hypoxic trimix (or an ROV) and you need to be able to carry a 50-100 kg payload in addition to all the diving kit. Just to qualify for the training courses to learn this type of diving you need to have logged many hundred dives with gradually increasing equipment complexity, it takes years and years of training.
I don't buy for a second that this is anything else than a nation-state actor with advanced equipment. The community of people in the world whou could dive to these depths and perform any kind of useful work is tiny and close-knit.
Loads of people I know did all that without any difficulty and without leaving their day job. They weren't in the navy either.
The most advanced thing that I see here is how do you trigger the explosion underwater, plus of course getting the explosives somewhere.
So an organization either needs to make preparations for this many years in advance, or try to recruit someone who both has some form of extremist beliefs and a hobby that requires very specialized training. Neither of those things are very simple, but the first one is doable, yes.