So- couple things.
320 feet is a cakewalk. Commercial mixed gas divers working simple DECO are working somewhere in the world 500+ right this second, I will guarantee it. It's not that hard. Saturation divers like @TLDR20
mentioned- much deeper. Huge surface supplied ships working ICW with subs and other submersibles can carry, transport and place hundreds of thousands of pounds of equipment. We worked with commercial divers in Scotland that routinely worked 400 feet- one of them worked a project at 900 feet... for 7 months.
The author has definitely published some bombshells that turned out to be true, he's also the guy that alleges the UBL raid was completely fictitious and didn't go the way that you were told, so I have a healthy dose of skepticism for him personally but...
... the timing links up. The fact that the regular Navy (and not SOCOM/JSOC) allegedly did the deed makes sense because JSOC would need express permission from Congress/SECDEF. Regular forces wouldn't. Navy divers are actually the right "tool" for this one- diving in SOCOM/SOF is almost always done for infil, not for salvage. Even for us (PJs) that have a distinct mission to recover... we tap the Navy when it's too deep.
I will hold what I got here- but you got motive, you got capability, you got timing, you got Biden explicitly saying that he was going to kill the pipeline during the exercise when it's alleged the explosives were being placed.
"If Russia invades, that means tanks and troops crossing the border of Ukraine again," Biden said. "Then there will be no longer a Nord Stream 2. We will bring an end to it."
When asked how, Biden said: "We will. I promise you. We will be able to do that."
That was Feb 2022.