Sure it becomes much simpler if you remove the diving aspect to it, or simply remove the need to return to the surface alive.

I would suspect dragging an explosive with a rope would take a long time, enough that radar and other ships would likely notice the behavior, especially if you intend to hit the pipeline 3 times on 3 different locations. Really expensive sonar would reduce the required time, as would really precise GPS locations, but even so, from what I know, it a very time consuming activity. Still a better theory than sport diving.

what about those remote vehicles they use for treasure hunting? The seem to be able to go down far deeper than what is being talked about here.

Drag explosives on a rope, but don't blow it up when it hits the pipeline - instead have it drop when that happens, and then detonate a week or more later.

Vessel positions are tracked and recorded after all.

Excellent counter to the idea that it must have been a coordinated effort with a sophisticated, well-funded adversary using submarines, and therefore Russia.

If they detonated it with a rope, they would have gone up in flames themselves as the radius of the leak is over 1 km long. It had to be a delayed charge.