Mystery Submarine (1963)
Director: C.M. Pennington-Richards
This is the VHS release (see more).
SummaryAn abandoned U-boat, complete with codebook and ship's log, is captured by the British and dispatched with a British crew to insinuate itself into a wolfpack at sea. The other U-boats discover the deception and attack the imposter, which wrecks itself on a reef while attempting to escape. The shipwrecked crew are then rescued by a British ship.
NotesAlso called Decoy.