From above, depth charges are cascading down, soon to explode around them. The men wince as one goes off. The sub rocks. Water pours in from some unseen wound in the submarine's hull.
Sailors scramble to plug the leak.
Call me a guy if you like. In the movie, the Nazis have brought the war to the United States' watery front door, floating up and down the eastern seaboard at will. So when the Americans pick up distress signals from a debilitated Nazi sub one day, they decide to send out a fake German rescue mission, and trick the crew into giving up its onboard decoding device. The point is to get the decoder before the crew has a chance to alert Berlin. Of course, they run into problems.