Revealing mistake: Almost at the end of the movie, the sub is being chased by a German destroyer. One of the destroyer cannon shots hits the stern portion of the U-Boat (where the diesel and electric motors are located). That hit should have instantly killed Tank (in the electric motors room) and maybe also Trigger (they are both working to fix the stern torpedo tube). This event reminds you that this is just a movie.
Revealing mistake: When the German destroyer is hit by the torpedo at the end of the film it is seen to stop dead in the water upon the torpedo impact. In reality momentum would have carried the ship forward some distance after the explosion. A thousand or so tons of steel just does not stop on a dime unless it hits something like a mountainside.