Carol Lynley's character Nonnie Parry's look was based on singer Renée Armand, who was the vocalist of the popular song "The Morning After" in this movie. She was also her stand-in:

The Poseidon Adventure - Renee Armand Interview:

Renée: "I was on my way to the beach, I was wearing my bikini cut-offs and that was it. And I got there, the costume lady or person took one look at me and said 'That's it, that's it, we'll use that', and Carol Lynley was then dressed for the rehearsal in like a bathing suit top and cut-offs from then on that little rehearsal thing that she does. And then they also hired me to stick around and sort of be a stand-in for her the rest of the time, which was lovely, but I never really did anything and just got paid to hang out on the twentieth century life. Very cool."

Side note:

So, Carol's singing voice was dubbed over by Renée's. This happens too often in movies, especially musicals - e.g. "My Fair Lady (1964)", "The Sound of Music (1965)" - when actors have a singing role and the producer(s) think that it's not good enough. They'll have a singer do that part.

It's a pity we'll never know how good Carol really sounded singing "The Morning After".