I've combined these two books since they are for all intents and purposes a single story, albeit one that takes a very swift turn in the middle.
I have seen the original Poseidon Adventure, Poseidon--the version with Kurt Russell, as well as a few terrible rip-offs. The main idea is the same in each, though there are a variety of causes and specifics. Basically, a giant cruise ship is rolled over into an upside-down nightmare, and a small group of surviving passengers have to journey through the topsy-turvy world in an attempt to make it to the bottom (now top) of the ship where their best chance at rescue lies. The first book is really pretty excellent--there is a decent amount of action, as well as the exploration of human dynamics, and the various ways people respond to crisis. Some, like the Reverend Scott, take charge and lead as though it were something they had been born to do. Others--like Dick Shelby, his family, Mrs. Kinsale the spinster, and the Rosens, an elderly Jewish couple--prefer to be followers. Some, like small haberdasher James Martin, only show their heroic nature when there's no other option. And then there's detective Mike Rogo and his low-class wife Linda, who seem determined to resist authority at every turn. It's a varied group who need to work together in order to survive. They have a number of challenges to face and they lose some of the group along the way. However, they always manage to keep hope alive. There are some big differences from the movie--I imagine they wanted to keep things a little lighter and more optimistic for the viewing public--but on the whole it's close to the film and is a good, engaging read.
The second book, Beyond the Poseidon Adventure was written as a sequel not to the first book, but to the film. The premise is that three of the survivors (Mr. Rosen, Mike Rogo, and James Martin) go back onto the Poseidon to protect the cargo Rogo was guarding on the trip, only to find themselves having to deal with leftover survivors, a noble tug captain and his daughter, a seductive thief, a mysterious American mercenary, and a danger Greek assassin. The action roams through the ship, and involves trickery, love, and even a tiger! The story is a little over the top, and the characters seem a little less natural in this one. It seemed clear to me that this sequel was a bit forced, and lacks the complicated personal dynamics of the first book, leaning more on the action to do the heavy lifting. Still, it was an entertaining way to spend a few hours on a chilly afternoon.
As a note, aside from the tug boat and the Greek assassin, the movie of the same name (starring Sir Michael Caine and Sally Field) bears only the slightest resemblance to the book. It's a very entertaining movie--better, perhaps, than the book--but I didn't want you to be fooled.