All of the internal submarine scenes were filmed inside an actual WW2 Submarine, being the USS Cod The USS COD Submarine Memorial is a National Historic Landmark and is docked in Cleveland, Ohio.
The aforementioned comment about the pistols being Walther PP's is incorrect. The pistols used were Mauser HSc's which were issued to Kriegsmarine and proof marked with an eagle over M.
At the 47 minute mark in the movie, U-578 is sunk by naval "hedgehog" depth charges in the Atlantic near coastal USA... but in reality, U-578 was sunk by allied aircraft attack at a different location in the Bay of Biscay near Spain with no survivors.
In this movie, a German U-Boot is shown in the Hudson River in New York, but in reality, only two U-Boots got so close, and neither entered the harbor. They only passed at the mouth of the river without entry, U-505 and U-37. Neither conducted hostile action in the Hudson during WW2.
At 17:10 the assembled German naval officers and sailors sing navy war song. This is taken from ""The Battle of the Bulge" (1965) when Col. Hessler meets an assembled group tankers who sing "Panzer Lied" -the German tank force song.