The British destroyer H23 is portrayed by the German escort destroyer (Flottenbegleiter) F7, which at this point served as a torpedo retrieval ship and was no longer a front-line warship. F7 was heavily modified cosmetically, with fake guns, torpedo tubes and heightened funnels to appear more like an actual warship.
The departing U-boat is cheered from the dock "dicke Beute!" which is correctly and literally subtitled "fat booty!".
The term U-boat (U-Boote) is short for "Unterseeboot" ('German for "under-sea-boat"). It can refer to any submarine, but is typically used for the designation of submarines operated by various versions of the German navy since the 19th century. The earliest designated U-boat class was the type U-1, commissioned in 1906.