[Stephen]

There was no master plan for the sequel, the studio just decided they want one. Interesting bit about the producers inital plot idea. It's from Wikipedia.

In 1973, soon after the first film came out, producer Irwin Allen proposed a sequel that would have had the survivors testifying in a hearing on the disaster in Austria, the country of the Poseidon's parent company. While on a train to the hearing, a miles-long mountain tunnel would collapse, leaving the survivors of the train trapped inside, struggling to make their way out. The film was planned to be released at Christmas 1974 from 20th Century Fox. Most of the main cast was initially intended to reprise their roles from the first film. This premise was eventually used in the Rob Cohen film Daylight (1996) with Sylvester Stallone.

Originally planned with the first film's distributor 20th Century Fox, the film ended up moving to Warner Bros. Pictures after they signed a three picture deal with Allen in 1975.

Relationship of Gallico's novels to the films
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Paul Gallico's novel The Poseidon Adventure had ended with the ship's sinking. The original film changed much of the novel's plot and ended with the ship still afloat. After the huge success of the film, Gallico was asked to write a novel that would be a sequel not to his first novel, but to the film. It would feature a new group of people entering the still capsized ship and could be made into a second film. In response, Gallico started writing Beyond the Poseidon Adventure, but he died on July 15, 1976, before completing the book. The book was published on January 1, 1978. Once again the film that followed ended up bearing little resemblance to his book. Instead of sinking, the ship explodes, along with Svevo and his men.