I consider this installment to be the least impressive of the four "Miss Marple" movies directed by George Pollock for MGM. Margaret Rutherford is excellent portraying the person in charge of a charitable organization focused on helping young people who’ve run afoul of the law, and Lionel Jeffries brings the humor as the Captain of "HMS Battledore"—the sailboat used to guide these troubled youths toward better behavior. When one of her colleagues unexpectedly dies—quite suddenly—"Miss Marple" believes something criminal has occurred, and with the assistance of some household chemicals (and, naturally, “Mr. Stringer”) determines that he was cleverly poisoned! She then heads to the coast to investigate the ship and uncover the truth, much to the annoyance of almost everyone involved. There's plenty of lighthearted fun by the ocean, and we even see a little sword fighting.

Source on TMDB