One of my professors when I was an anesthesiology resident was ex-Navy. He had been in the submarine service, and not as just a general medical officer. As an anesthesiologist. You can imagine the kind of missions a submarine with an anesthesiologist on board might be sent on. There might even be a surgeon on board...
Anyway, he was dive-rated. Closed-circuit breathing is the basis of anesthesia machines and has been for ages (at least 60 years). So for us, a closed-circuit system isn't new - it's something we do every day at work. You just have to account for depth. I'd be shocked if he hadn't done it.
That is cool as hell. And yes, one can imagine. The list of boats he was probably on is extremely short: Seawolf (the 2nd one), Halibut, Parche, and Jimmy Carter.
Incidentally, I would add that while he never specified, he very likely may have shipped out as a “general medical officer” with the crew not necessarily knowing he had a deeper skill set. They wouldn’t necessarily twig to it on early deployment. Only when they take on a SEAL team and a surgeon would it become obvious (or he goes on another boat with a surgeon as the GMO). The only obvious clue would have been his being slightly older than a typical GMO.
Anyway, he was dive-rated. Closed-circuit breathing is the basis of anesthesia machines and has been for ages (at least 60 years). So for us, a closed-circuit system isn't new - it's something we do every day at work. You just have to account for depth. I'd be shocked if he hadn't done it.