If I recall correctly, Steiner and his men have been assigned to do suicide missions as punishment for some misbehavior / disobedience towards a superior officer. They are doing something with midget / one man submersibles like the Neger human torpedoes, a VERY hazardous duty with something like an 80% fatality rate.
Been quite awhile since I saw the movie, will have to refresh my memory again soon.
Hope this helps!
Update after looking at wikipedia article: “Second he selects Kurt Steiner, a highly decorated and experienced Fallschirmjäger officer, to lead the mission. However, while the Luftwaffe parachute troops are returning from the Eastern Front, Steiner unsuccessfully attempts to save the life of a Jewish girl who is trying to escape from the SS in occupied Poland. He and his loyal men are all court-martialled and sent to a penal unit on German-occupied Alderney where their mission is to conduct near-suicidal human-torpedo attacks against Allied shipping in the English Channel.”