In the original, the helicopter rescuing survivors is a Sikorsky S-55 variant but this helicopter becomes a Sikorsky S-61 variant in the sequel.
When Mike and Celeste get back to the tug, their yellow scuba tanks suddenly disappear, but they save their masks and swim fins.
When the group comes to a passage blocked by furniture, they remove the same silver-painted wooden stool from the pile twice.
After the storm hits the tugboat Jenny, one of the coffee mugs being used keeps disappearing and reappearing from a hook on the bottom of the shelf over Karl Malden's right shoulder.
When Telly Savalas is about to climb out of the Poseidon through the hole that was cut in the bottom of the ship in the first movie, the hole is round shaped, but when he emerges from the hole on the outside, the hole is more rectangular.
In the first film the survivors are clearly being followed deck by deck by massive amounts of sea water which are flooding the ship surmising that all decks below them are under water. In this film the salvage crew penetrate deep within the ships hull onto decks which seem not to be flooded.
In the first movie, The Poseidon Adventure, the ship capsizes due to an undersea earthquake which generates a tidal wave that hits the ship. In this film the Poseidon is portrayed as capsizing due to a wave generated by a massive storm in the Mediterranean Sea.
The hull of a 40-year-old ship that's about to be scrapped would not be shiny bright red.
There are no barnacles on the upturned hull. The bottom is pristine, looks as if it just slid down the ways.
When the party is swimming to the surface, they're each wearing weight belts. While these might provide some marginal value to the swimmers not wearing SCUBA tanks while swimming horizontally through the ship, the weight belts would make it very difficult for them to swim to, and remain on, the surface.
The ships hull for the sea scenes was built only slightly bigger than was visible above the water line. To continue the illusion under the water line the hull had a fabric skirt that was painted red which fades to blue. This fabric skirt can be seen at the end of the movie when Capt Turner dives back underwater to find Wilbur.
Tex opens a panel in the wall, revealing a raging fire. In fact, he couldn't open the panel, as it would be too hot.
After the automatic gunfight, from the cardeck to the corridor, a view of the corridor outside the hatch reveals no bullet marks where the gunfire was aimed at.
Inside the ship there are fires everywhere, but no smoke. The fires have been burning for a long time, but nobody is affected by the heat. The engines have stopped, but regular light fixtures remain lit. The hole(s) cut in the bottom of the hull would have let trapped air out that was keeping the ship afloat, causing it to sink faster.
Only 50 metres (approximately) of the hull of the Poseidon remains above water when boarded by the salvage/medical team because the front of the ship is sinking lower and lower into the water. Yet all the decks inside the ship are horizontal. For the decks to be horizontal, the ship would have to be sitting level in the water.
In the Poseidon's cargo hold we see three vehicles; a 1975 Jeep CJ-5, a 1975 Triumph Spitfire, and a 1977 AMC Gremlin. The Poseidon capsized in 1972, three years before the oldest of these cars were built.
During the firefight, a couple of the flash effects depicting ricochets off metal goes off prematurely or too late and no corresponding gunshot is heard.
Equipment shadow on Wilbur's back when the crew first meets Svavo.
In The Poseidon Adventure the ship capsized due to an underwater earthquake that triggered a huge rogue wave. In this film, however, the ship is capsized by a wave due to being in a fierce storm.
The tugboat Jenny and the Poseidon are portrayed as fighting the same massive storm in the Mediterranean at the beginning of the film. However, were as the Poseidon is capsized by a huge rouge wave, the Jenny encounters no such wave.
Nurse Gina Rose tells Karl Malden's character Wilbur that she left a job as a nurse in Philadelphia and decided to change careers by signing onto the Poseidon as the ships nurse to add a little spice to her life. However, the liner as explained in the first movie was on her last voyage on the way to the scrap yard. Thus after only one voyage Rowe would be out of the job.
The party has to avoid some hanging electrical wires outside the purser's office, and use a broken railing to hold the wires them to one side so they can pass. However, the person holding the wires back touches them anyway without being shocked.
When Captain Turner (Michael Caine) moors his boat (The Jenny) to the Poseidon, and then boards the sinking Poseidon, he should have been concerned that if the Poseidon continues to fully sink, it would take his boat down with it.