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.
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.
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.
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.
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.
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.