Hey, those who play 100%, DiD don't do that
But seriously, I would have to acknowledge a bit from both perspectives here - yes, reality is nothing like a game. On the other hand, a game can get you a lot closer than one might assume. There's nothing natural about dozens of men stuck in a steel tube performing complicated maneuvers, and if a game can get you into a mindset where you have a good understanding of how the steel tube works and how best to use it and not use it, that is some serious training. Likewise, on the point of staying calm - you'd be surprised how much games can assist with that. Any high-stress situation is made easier when you are in the mindset where a) you know what is happening; b) your mind and body have a good idea of what needs to be done about it. A simulator definitely gets you at least halfway there, and while it certainly does not get you to the level of discipline required, it does get to a sort of starting point for it. The rest would come from drills in an actual sub. I mean let's face it, any real submariner also has their first real encounter in combat. And as an SH4 player, you know what? You already probably have a better understanding of what to expect from combat than they did.
Real submariners aren't made of different stuff than you are. The real difference is that they are drilled better, and that's not something that some training wouldn't fix in the average SH4 player. Most WWII submariners were average Joes too, and when they enlisted, most of them did not have nearly as much submarine knowledge as you might have. Now, noone would let a submarine nerd sail on a real submarine without training of course - so I think the real question should be "Would you enlist in submarines if you lived in WWII?" And I think the real answer is that guys with as much knowledge as you get from playing SH4 would make pretty darn good candidates.