The real HMS Amethyst was used at the start of filming. A special effects explosion was made too big and blew a hole in the hull, flooding the engine room. Amethyst was sent to the breakers and HMS Magpie was used for the rest of the film.
The River Orwell, which runs between Felixstowe and Ipswich in Suffolk, England doubled as the Yangtze River during the making of this film.
HMS Amethyst was brought out of storage to participate in the film.
The black-and-white feline seen aboard the Amethyst represents Able Seacat Simon, the ship's cat who became a celebrity after receiving the PDSA's Dickin Medal (at the behest of Lt. Cmdr. Kerans) for raising morale, despatching a rat infestation and surviving cannon shell injuries. The entire Amethyst crew attended his funeral in 1949 following his death of a viral infection caused by his war wounds.
The technical adviser was Commander John Simon Kerans, the British Naval Attaché "who commanded H.M.S. Amethyst during much of the period of the story, and whose exceptional help is gratefully acknowledged" as the credits put it, (He is portrayed by Richard Todd). He had been awarded the DSO Distinguished Service Order ("for distinguished services during active operations against the enemy") for his part in the Amethyst incident, and soon after helping on the film served a term in the House of Commons after being elected Conservative Member of Parliament for The Hartlepools district from 1959 to 1964.