Red Buttons and Carol Lynley, whose characters fall in love in the movie, actually disliked each other intensely during filming. They refused to have anything to do with each other except when the cameras were rolling. Ironically, after being constantly reminded of this, they ended up becoming great friends in later years. Both Carol Lynley and Pamela Sue Martin were with Red Buttons at the time of his final public appearance -- the world premiere of Wolfgang Peterson's "Poseidon" at Mann's Chinese Theatre in May 2006.
The original script called for Rev. Scott to send Mrs. Rosen on her underwater mission, and for her to be trapped and need rescuing by him. Gene Hackman decided that his character would never ask her to do this, and suggested their characters' situations be reversed. Director Ronald Neame agreed, and they persuaded Shelley Winters that this was indeed better for her character.
Paul Gallico was inspired to write his novel by a voyage he made on the Queen Mary. When he was having breakfast in the dining room, the liner was hit by a large wave, sending people and furniture crashing to the other side of the vessel. He was further inspired by a true incident which occurred aboard the Queen Mary during World War II. Packed with American troops bound for Europe, the ship was struck by a gargantuan freak wave in the North Atlantic. It was calculated that if the ship had rolled another five inches, she would have capsized like the Poseidon.
Except for the most dangerous sequences, all of the stunts were done by the actors themselves. All the actors at one point complained to the production staff about how difficult the shoot was physically.
An ending scene showing rescue boats surrounding the sinking ship was planned, but the budget ran out. The shot of the helicopter lifting off the hull was done on the studio lot, looking upward to avoid seeing the surrounding buildings.
Most of the external shots of the Poseidon were shot using a model built from the original blueprints of the Queen Mary. The model is on display at the Los Angeles Maritime Museum at the Los Angeles harbor. The real Queen Mary is located just a few miles away in Long Beach.
Filming was delayed twice because of the cost. Twentieth Century Fox was suffering from losses from several flop musicals, Doctor Dolittle (1967), Star! (1968), and Hello, Dolly! (1969), as spectacles were being trounced by smaller character-driven films, and the studio was certain that a disaster movie would be a risk. Fox finally relented when Irwin Allen promised to raise half of the budget himself. Reportedly, Allen found outside backers by walking across the street from the Fox lot to a country club, where he found some friends playing cards. During the card game, they agreed to back the film. Because the studio never spent any of the backer's money, the backers made a profit from the success of the film without actually spending a dime.
Such mid-ocean "rogue waves" were previously thought to occur only once every ten thousand years. A 2004 study of satellite radar images showed they can happen as often as hundreds of times every decade.
In the scene in which Rev. Scott rescues Robin, the set was built on tracks which would slowly lower the inclined set into a large water tank. The set was supposed to stop moving once the set was half-submerged, but for some reason it continued until the camera crew was underwater. The film magazine was rushed to the lab, where immediate processing showed the film was undamaged.
Some of the pre-capsize sequences were shot aboard the Queen Mary, including the opening storm sequence, the pre-disaster scenes in the staterooms and hallways, the scenes above decks, and an early scene in the engine room.
The set for the banquet hall was designed so that very few objects needed to be moved from the floor to the ceiling (and vice versa); the columns along the walls were identical at the top and bottom, and the wall decorations were all removable.
The famous interior "capsize" scene was done in two parts. The first part had the hydraulically controlled set tilted to its maximum 45 degrees. The cameras were then stopped and the set was redressed so that the floor (deck) became the ceiling (overhead) and vice-versa. The actors were then returned to the set and the set was again tilted to complete the sequence.
In the scene where Rev. Frank Scott is giving his sermon on the deck, actress Pamela Sue Martin is wearing a white and yellow poncho that was actually made for actress Rosemary Forsyth who wore it in City Beneath the Sea (1971), the TV-film that Irwin Allen produced the year before.
In her autobiography Esther Williams claims she was offered the role of Belle Rosen by producer Irwin Allen because of her former swimming roles (though this remains open to debate, as the character of Belle Rosen called for a large woman).
The sequence where Nonnie (Carol Lynley) rehearses "The Morning After" with her band mates was the first scene to be filmed. Originally Waddy Wachtel (the guitarist) was to be cast as her brother Teddy, but as Wachtel had brown eyes and Lynley was blue-eyed, drummer Stuart Perry was cast as Nonnie's brother.
The trivia item below may give away important plot points.
In the novelization there are several other characters who go up the Christmas tree with the survivors. Towards the end it's shown there was a second rescue party with some of the survivors - including the nurse - from a second section of the ship.