At the start of WWII, the German fleet was much too small to directly challenge an opponent to war. Throughout the war, Germans used submarine warfare more than any other kind of warfare. One of the ships used with the Germans, was called the U-boat. The term could be used for any of the German boats, but to the English, it was mainly used to title the military submarines. Down below is a video on a German U-boat in action.
Torpedoes, as shown in the video, were the German's weapon of choice. By the end of the war almost 3,000 Allied ships were sunk by German torpedoes. Torpedoes would often run at an improper depth, not explode at all, or explode too early- leading them to bounce of a ship harmlessly. Many of these kind of things occurred due to a lack of testing. Over time the U-boats developed until they were extremely strong and powerful.