A remake of the 70s disaster film. From what I remember, it is pretty faithful to the original, only made with more modern means (and sporting a shorter runtime, which is always a plus). It's not the most memorable or spectacular film, but if you like a good disaster flick then it's pretty amusing.
The Poseidon is a huge luxury cruise ship. On New Year's Eve, it is hit by a rogue wave, leaving it capsized in the middle of the ocean. There is still air trapped inside the vessel, but time is running out for the passengers who survived the first hit. The boat is slowly sinking and there's no outside help.
Some B-list (or former A-list) actors make up most of the cast, but this film is more about the special effects and set pieces anyway. It's nothing you haven't seen before, and because the film is quite short the pacing is slick (no time for too much unnecessary drama along the way). Pretty decent.