Murder Ahoy! (1964)

When Cecil Ffolly-Hardwicke, co-owner of an ex-naval ship, dies suddenly, a string of similar deaths follows. Luckily, amateur sleuth Miss Marple (Margaret Rutherford) is around to investigate, deducing that Cecil was poisoned. Marple's further digging finds that the battleship -- ostensibly being used to rehabilitate troubled boys -- actually serves as a criminal training outfit run by the corrupt Lt. Compton (Francis Matthews). But is Compton the murderer, or is there more to the mystery?