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.
Dying 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.
Irony as She Is Cast: Carol Lynley, who played terrified non-swimmer Nonnie Parry, is actually an avid swimmer in real life.
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.
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.
Gene Wilder was offered the role of James Martin, but had to turn it down due to a scheduling conflict.
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...