Elektroboot

An Elektroboot ("Electric Boat" in German) was the first submarines designed to operate entirely submerged, rather than as submersibles that could submerge as a temporary means to escape detection or launch an attack.

Elektroboot in Worldwar
An 'Elektroboot was used to deliver Germany's third atomic bomb to Alexandria as the submarine sailed from Crete to Alexandria completely underwater. When Heinrich Jager heard the news he thoroughly believed that if the Lizards hadn't come they would've swept the ocean's clean of allied shipping.