What makes you doubt Hersh's story?
So the situation is essentially "Hersh claims that someone told him a story that we know is at least slightly untrue." Is it possible that the rest of it is true? Absolutely!
...but there's nothing I've seen to suggest that it's likely the rest of it is true. It's just another theory, of which there are many.
Also he only quotes a single anonymous source. He has stated before that he verifies info from multiple sources, and goes to great lengths to protect sources, so it is very likely that there are more but that he cannot give details without endangering them.
https://twitter.com/MarkAmesExiled/status/162628975702088089...
There's a few, but as an example, Hersh claims the divers deployed off of an Alta-class minesweeper, which was taking part of the BALTOPS22 excercise.
There was no Alta-class minsweeper taking part in the excercises. And while a similar Oksøy-class minesweeper did take part, it's AIS track doesn't put it in the right spot. When Hersh was challenged on this detail in an interview, he said "It was called the Alta, the ship was there. I mean that is just such a stupid lie." suggesting he meant that the ship was called the Alta, not that it was an Alta-class ship. But the Alta is an Alta-class ship, and also it was in the process of being scrapped at the time, and also wasin't taking part in the excercise.
None of that means that divers didn't deploy off some ship at some point! But Hersh provided very specific details at what ship it was, when it happened, and how it happened, and those details are wrong, which raises the question of what else was wrong. Was it a minsweeper? Was it even actually a Norwegian ship? Was it actually during BALTOPS22? Did the incorrect details come from Hersh or from his source? If the source, was that the source trying to cover their tracks, or did the source, perhaps, not have direct knowledge either, but was just retelling a story they had heard? Etc.
...but he also seems to have gotten it wrong in some other cases. Hersh is a fantastic investigative journalist (and a good writer and speaker to boot), but he has very limited credibility because he will make fantastic claims regardless of whether he has the ability to back them up.
I mentioned them in one of my other comments but this is a decent (but long) roundup of Hersh's history: https://nymag.com/nymetro/news/people/features/11719/
Or for quick summary (but without a lot of the details) see: https://talkingpointsmemo.com/edblog/whats-the-story-with-se...
Notably there doesn't appear to be any reason for him to claim it was a single source when he could equally have claimed he verified it with multiple sources (even if there was only one additional source. It's no less untrue, right?)
I think in these circumstances you can only look at Hersh's record. Since the turn of the millennium it really isn't very good. With the exception of the Abu Ghraib story (which he wasn't alone in breaking) I don't think any of his major stories have turned out to be true: https://en.wikipedia.org/wiki/Seymour_Hersh
(He was also very lucky to avoid publishing forged claims about Kennedy in the late 1990's).
Or for a shorter version (but the same point): https://talkingpointsmemo.com/edblog/whats-the-story-with-se...
Hersh always benefited hugely from having a very good editor to keep him disciplined and reign in his enthusiasm.
The Wikipedia article quotes a number of journalists and editors who make similar points.
It's interesting you question his record, I thought Hersh's record was excellent; which major stories are you referring to? With the kind of reporting he does, I would have expected many more major mistakes to be made but he seems to make them very rarely.
I linked Wikipedia, but to summarize some of his questionable claims:
* There were plans under consideration in 2006 for a nuclear first strike on Iran.
* Osama bin Laden wasn't killed by US Forces in a raid in Pakistan
* The Syrian gas attack wasn't done by Syrian government forces
* "I don't necessarily buy the story that Bin Laden was responsible for 9/11"
* That the Skripal poisoning wasn't done by Russian forces.
Lots of these views have significant backers amongst conspiracy theorists. But unlike some of his other stories (say in the same time frame) there has been very little to no evidence showing he is anything like correct.
Basically, post 2000 the only one of his stories which has turned out to be well supported was Abu Ghraib.
Classified info takes time to be released, so I wonder if the fact that his recent reporting hasn't been confirmed is more a function of freedom of information / declassification timelines than proof that he has dropped the ball.
* That source is implausibly well informed.
* Operational details make little sense (using a warship as a diving platform for US divers, elaborate sonobuoy remote detonation, etc)
* OSINT details don't check out (all the implicated ships are accounted for, etc)
* Errors/contradictions in the story (the Norwegian Navy doesn't operate any P8s, etc)
* Long, unbroken track record, going back to 2006, of Hersh writing lurid, discredited stories like this
* No media outlet in the world --- not just in the US but anywhere in Europe --- being willing to run this.
Important caveat that disbelieving Hersh doesn't mean that the US wasn't behind the incident; it just means there's nothing to learn about the incident from Hersh's writing.