"Run! Come on!" the captain shouted from the sail bridge of the Type XXI U-Boat. The agents they were supposed to pick up were running like hell down the pier, their pursuers were still by the warehouse. The U-boat sat untethered, the captain had guided the helmsman as close as he could. The agents leaped the gap and landed on top of the sail, scrabbling inside.
They heard the sharp reports of gunshots and the sides of the sail sparked and pinged. The captain banged on the door of the gun turret in front of him. "Your ten 'o clock, return fire!" In response the turret turned and the twin-barrel 20mm FlaK gun lit up the night as it chattered back. "Back emergency!" the captain shouted into the comms tube. The people on the sail heard the idling diesel engines roar with power and the sub began to move backwards away from the pier. The agents' assailants were taking cover behind a building, and the FlaK gun was tearing the area apart, tracers were striking all around them.
They maneuvered the boat around and went tearing out of the bay at flank speed, making around 16 knots.
Just as the captain predicted, two speedboats came roaring after them. The aft FlaK gun opened up, trying to ward them off. A bullet hit the engine of one, stopping it cold. But the other boat kept coming. The captain ordered everyone inside and the hatches locked. The speedboat came along side and its occupants jumped onto the U-boat. But when they tried to get inside, they found they couldn't budge the hatches.
"Dammit!" one said. It'll take a cutting torch and twenty minutes to get in there!
"By then they'll be gone; we can't let them escape. Keep trying!" said the other
They struggled for fifteen minutes straight before one of them looked over the side of the sail bridge.
"Holy-- we're a long way from the harbor! They could dive at any minute!"
As if on cue:
AAWOOOGA! AAWOOOGA!
The decks below the sail began to sink under the water. There was a smooth hissing and the forward dive planes swung out from their slots.
They began to panic when water began to fill the area they were standing in.
"Come on! We have to get off here!" He vaulted the side and stood on the plating. His partner attempted the same, but somehow got his leg caught on the radar ring.
"Hey! I'm stuck!!"
His partner tried to pull him free, but he was stuck fast. He was forced to let go as the sub dragged his partner and friend down to the crushing depths.