Creator In-Joke: Robin jokes about almost dying in the bathroom, and Susan jokes it would be a silly way to die. In the novel, Robin is last heard of when he's going to the bathroom.
Dawson Casting: In Beyond the Poseidon Adventure, 26-year-old Angela Cartwright plays a teenager, signified by her "innocent" 1950s-style wardrobe. Inverted with Peter Boyle (b.1935) as her father, playing a World War Two vet, despite being only ten when the war ended.
Dyeing for Your Art: Shelley Winters gained 35 pounds for the part of Belle Rosen. Afterward, she complained that she was never able to get back to her original weight, no matter how hard she tried. She also trained with an Olympic swimming coach so that Mrs Rosen would come across as a natural in the water.
Hostility on the Set: 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.
Irony as She Is Cast: Carol Lynley, who played terrified non-swimmer Nonnie Parry, is actually an avid swimmer in real life.
No Stunt Double: Most of the actors did their own stunts themselves, in fact complaining to the studio about how physical the shoot was.
Non-Singing Voice: The Award-Bait Song "The Morning After" is sung by Carol Lynley's character Nonnie, but Lynley's voice is dubbed over by singer Renee Armand.
Prop Recycling: In the scene where Rev. Frank Scott is giving his sermon on the deck, 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 Made-for-TV Movie that Irwin Allen produced the year before.
Real Life Writes the Plot: The ending was supposed to show rescue boats surrounding the sinking ship but the budget ran out. The shot of the helicopter lifting off the hull with the six survivors was done on the studio lot.
Urban Legend of Zelda: Esther Williams, in her autobiography, claims that she was offered the role of Belle Rosen because she had done many swimming roles in the past. This is open for debate, as Mrs. Rosen was specifically written to be a large woman.
Wag the Director: The original script called for Rev. Scott to send Mrs. Rosen on her underwater mission, and for her to be trapped and needing 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.
Waddy Watchel, who plays the guitarist in Nonnie's band, was meant to be cast as her brother. But because the two actors had different coloured eyes, Stuart Perry (the drummer) became her brother instead.
Gene Wilder was offered the role of James Martin, but had to turn it down due to a scheduling conflict.
Irwin Allen commissioned Paul Gallico to write a novelised sequel to the film before he died. The resulting Beyond the Poseidon Adventure was ignored for the eventual film. The book features Rogo and Manny returning to the Poseidon to find their wives' corpses, Martin tagging along to prove he is a man. Manny is brutally gunned down by thugs hired by the ship's owner. Rogo would be revealed to have been on board doing work for the CIA, guarding plutonium. New characters would include Hungarian pirate captain Bela, a teenage orphan girl named Coby, and an albino adventuress named Heloise.
A rival attempt by Fox to create a sequel may have influenced Allen's decision to not include any original cast. Their idea was Atlantis: Poseidon II, set five years later, beginning with the Reverend Scott's vengeful sea captain brother leading a raiding party on the sunken, abandoned Poseidon. There, they find robot manta rays created by the Poseidon's owner Nikaros to guard the wreck. When one of the party is killed and the Poseidon all but vaporised by the exploding manta rays, Scott goes out on an all-out revenge quest to find what Nikaros is hiding. He tracks down Rogo, now living an unhappy simple life in Rome, mostly retired, but a Mafia informant on the side, and together they work out an uneasy alliance. We learn that James and Nonie are married and unwilling to help, and Manny Rosen raising his grandchildren in Tel Aviv, so the only other people around to help are Robin and Susan Shelby, now living in England with their mother and stepfather, and Robin a Royal Navy cadet. Together, they would uncover that Nikaros is planning to build a floating superhotel, the Atlantis, on the site of the Poseidon's wreck, a fault line that will lead to catastrophe once more, especially as a North Sea oil platform lies on the same line. A vengeful Ahab-ish Scott would try to bring down the Atlantis before it is opened, his plan bringing the unauthorised construction to the British, French and Italian authorities. Nikaros, resilient throughout thinks he's doing the right thing becomes engaged in his own battle to keep the Atlantis afloat, eventually holding rich and famous European aristocrats hostage there. More people start to gather for Nikaros' party, until it is too late, and the unfinished Atlantis collapses, Scott, Rogo, Robin and Susan leading a gargantuan Titanic/Dunkirk-type rescue attempt. It would have been more of a spy thriller, ending with a helicopter gunship attacking the in-construction Atlantis. The Spy Who Loved Me and Orca perhaps killed this incarnation.
Another pitched sequel idea, according to The Other Wiki, would have had the group from the original film traveling to testify at an inquest about the ship's sinking. They would have become trapped in a tunnel collapse and had to navigate their way out much like when they were trapped on the ship.
Write What You Know: Paul Gallico was aboard the Queen Mary when she was hit broadside by a rogue wave during a storm off the Scottish coast in 1942. The ship listed 52 degrees before righting herself; had the wave been even a little stronger, she would have capsized. From there, Gallico started thinking about characters in that particular situation...