Those men were frighteningly brave. That thing was a deathtrap,,,with almost no hope of escape.
The H.L. Hunley was basically made out of a boiler. As a teenager I took a summer job at a boiler-works. Among the slightly less prestigious tasks was removing the slag after the ends were welded on.
In order to do that I had to enter the boiler through a tiny hatchway. Claustrophobia always got the better of me and I would bail after 5-10 minutes. I can’t imagine going underwater in one.