The film was shot mainly aboard a real ocean liner. The Hamburg had recently been sold by its German owners to the Soviet Union. Before the Soviets took delivery of the liner, they rented it to the film company. The liner was painted in the livery of a fictional shipping line, very similar to the livery used by the Soviet Morpasflot line, and renamed the Britannic. Advertisements were run in British papers, soliciting extras who would take a lengthy cruise in the North Sea for free, but with the knowledge that the ship would actually seek out the worst possible weather, as the story demanded seas too rough for the lifeboats to be lowered, trapping the passengers on board.
The film took its inspiration from an incident which occurred in 1972, when a man claimed he had planted a bomb on-board the QE2, and demanded a ransom. Cunard were prepared to pay the ransom, but the British Government decided instead to send in soldiers of the Special Boat Service, who parachuted into the North Atlantic one thousand miles from the UK, and boarded the QE2 to search for the device. The threat turned out to be a hoax, and the F.B.I. later caught the culprit.
The project was originally to be directed by Bryan Forbes. After his departure, Don Taylor was hired, but departed four weeks before shooting was to begin. Richard Lester then came on-board. He re-wrote the script with Alan Plater. Writer and Producer Richard Alan Simmons was unhappy with the changes, and had his name taken off the film, choosing to be credited as "Richard de Koker".
There were grave concerns when filming commenced without Sir Richard Harris. Harris was in New York City, and the crew had to schedule filming around him, with the producers worried that he wasn't going to turn up at all.