There are many categories of damage that simply cannot be fixed at sea. They require a drydock. That especially includes hull damage, which cannot even be fixed by resupplying on shore. You must end your patrol and let the boat go to drydock for that kind of repairs.
In real life the sub would be considerably more fragile than what you find unbearable. A single hole in the pressure hull would render the boat unable to dive and that condition would probably result in abandoning ship because it has become a target. The damage you are taking is much more severe than a single hole in the pressure hull.
A submarine is not some kind of little battleship. You are not Rambo. If you ARE Rambo, you are Rambo armed with a squirt gun in your underwear. It's best not to be seen. You should not allow any plane to even see you, much less bomb you.
A submarine is not an icebreaker. Ramming things sinks you and makes those on the vessel rammed laugh at you. Don't do that.
Even merchant ships have guns on stabilized platforms with functioning fire control systems. You have an unstabilized clothes drying rack with a couple of grooves in the barrel to sight with. Pointing your finger at the enemy and yelling "bang! bang!" is quicker and more lethal. Use the deck gun to dry laundry.
There is one reason Prein succeeded. What he did was impossible. Therefore his attack was not expected. Even that was not enough without strike after strike of incredible luck. The odds that he would end up feeding the fishies was much greater than that he would sink anything at all.
Similarly, you can only succeed in very unfair fights, where you are never detected, where the enemy is completely unaware and unsuspecting of your presence, and where you have a good deal of good luck.
You're thinking a submarine is some sort of super weapon out of Star Wars. It's more like a flintlock blunderbuss.
