You have already found a disguise, acquired level one papers and destroyed the Naxos Prototype.  Now, you're going to enter the pen (pic. 1).  You'll have to show your papers and find the U-boat.  But before, you must acquire level two papers to be able to pass the guard near the U-boat.

    Just out of a curiosity, although you may already know it, you can get your papers by just pistol whipping the germans, when they are alone (pic. 2).   You can save some bullets and it's a lot funnier to beat them than just shooting them once.  To pistol whip a german, just stand in front of him, press "1" and right after press the second function key.  Then, pistol whip him twice more because it takes three to kill him.  Of course that before pistol whipping any enemy, your weapon should be holstered.

    After you show your level two papers to the guard, you must head to the submarine.  But, DO NOT enter inside it now. You have an important last job to do.  You have to kill the two german workers on the submarine, outside of it.  Why to kill them if you're in a stealthy mission?  Simply put because if you don't easily kill them now, you'll have to deal with them later, in an extremely more critical situation.  The second worker (pic. 3) can hit you even if you're crouched on the uppermost deck.  So, go behind the deck of the submarine, select your hi-standard silenced pistol (I love this weapon!) and kill both.  The other enemies, far away, won't notice.

    Before trying to get out, you must know what you're going to face outside or you don't stand a chance of escaping.  Since you've already killed the two german workers, there will be four enemies outside, waiting for you (these four germans will always be there, no matter how many more enemies you eventually killed before getting inside the U-boat).  Two germans will be on the dock alongside the sub, to your right: one behind you, carrying a machine gun; the other ahead of you, carrying a pistol (pic. 5).  Two more germans will be far ahead of you, beyond the dock, carrying rifles.

    Time to scuttle.  Quickload with the forward key already pressed.  Keep pressing the forward key until you're out of the sub (after the black screen) and, as soon as possible, stop pressing forward and CROUCH.  You're at the uppermost deck of the sub, you must get out of there.  But, before, you have to kill all the four enemies.  Your health should be 100%, when you crouched, and you can kill them all and still keep it 100% !!  Note that you have to get used to the controls.  I found it difficult to crouch without first getting hit because it looked like I was always sort of stuck in the hatch but after training a few times, you will be able to do this rather easily.

    As soon as you crouched, turn to your right and walk ahead a little.  Quickly stand up and kill the enemy with the machine gun, head shot, no time to waste (pic. 6).  Crouch again quickly.  Head back to the position you were before, change weapon to the Springfield on your way.

    With the Springfield ready, stand up, select the scope and aim the german far away, straight ahead of you, the one to the left.  One head shot to kill him.  Immediately after, aim the next one, to the right: another head shot and he's gone (pic. 7).

    Without stopping, aim again to the right, at the dock, and shoot the german with the pistol (pic. 8).  After you kill this last enemy, you should have around 10-14 seconds to get out and reach the dock.

    Holster your weapon, jump down from the uppermost deck, head to the ramp and climb it up to the dock.  You must be completely out of the ramp by the time the U-boat explodes or you'll be dead.  If you're quick enough, at the dock turn yourself back and take a look at what you've done: the submarine explodes; the ramp is vanished (pic. 9).