...a good script or director couldn't fix.
The original 'Poseidon Adventure' was a story of human courage triumphing against terrific odds and personal tragedy. The survivors were led by a charismatic figure of great spiritual strength who would take anything God threw at him.
The follow-up tries to recreate the mix but fails through a formulaic script and pedestrian direction. Irwin Allen may have been a great producer of disaster flicks, and done a fine job directing the action scenes in 'Towering Inferno', but he just can't bring any human depth to his characters. If the characters aren't credible any danger they face also falls flat.
The script also tries to copy the original too obviously. So we have Peter Boyle doing the Ernest Borgnine thing by being tough and obnoxious (but he has a tender heart); Karl Malden is the Red Buttons moderating influence (and is terminally ill for good measure); Slim Pickens does the comic relief for Shelley Winters, and so on.
To make the story more contemporary we have a rogue arms dealer ready to flog weapons-grade plutonium to the highest bidder. A really nasty piece of work who ruthlessly abandons wounded men (the actors playing his henchmen were presumably paid as extras because they don't seem to have any lines) and has a woman shot in the back - what a cad! Thank goodness the French rescue services made the hole in the Poseidon's hull twice as large as in 1972 (and on the other side of the propeller shaft) so he could he get his goods out. And while we're on the subject, how did they get the crates *to* the... oh, forget it.
I actually paid good money to see this when it was released. Given the film's current reputation this may seem odd, but it actually got quite mixed reviews at the time. Some said it was junk, some said it was as exciting as the original. Never mind, nobody can be right all the time.