In the first Poseidon, after the ship turns over, leading hero Gene Hackman tries to talk all the trapped human/rats in the ballroom into following him up the giant Christmas Tree (rent the movie!), upward toward safety at what is now the bottom of the boat. He gives a big speech to the multitude, exhorting them to follow him to safety. They refuse to follow and are more or less immediately wiped out by great surging waters breaking through giant panes of glass into the hall of luxury in which these foolish mortals partied while nature gathered her forces to teach them a lesson.
But in the second Poseidon, the one that's now playing at your local multiplex, leading hero Josh Lucas and fellow hero Kurt Russell improvise a little group of accidental neighbors and bottom-line family members to make the ascent to safety. There is not even a hint in the script, or in any shot or musical interval, of responsibility for the whole population of that doomed world. The select few just move out on their own. Everyone else is an extra, in concept as well as fact.
The conscious decison of the movie makers was probably, hey, let's be realistic here.