This is the German term that describes the submergible watercraft that they developed and used extensively for World War I.

U-Boat (Unterseeboot)

Explanation:

During World War I, German Submarines played an essential role in the North Sea and the war. Germans were quite advanced when it came to controlling submarines. During the first World War, commander Franz Becker guided German submarines which we know today as U-boats. The Germans unrestricted submarine warfare tactics caused great offence to the merchant vessels in the waters around Great Britain particularly of the Royal Navy and America.