Robert Ramsey exhibits behaviour that indicates why he "used to be a fireman", as he knowingly stands right in front of a hot door while Dylan Johns opens it, and fails to check if the extinguisher he's going to use even works.
The experienced fire fighter grabs the door handle briefly with his hands to see if it was hot. It is taught in elementary schools to touch the hot door (or in this case the door handle) with the back of your hand. This prevents burns from limiting the use of your hand.
At 1:02:47, after Elena has hit her head underwater and is bleeding out, you can see her hand attempt to move her hair from in front of her face just before she and Robert break the surface of the water. When they surface, she is obviously unconscious.
While in the ventilation shaft, Robert asks the group below for a ring or a coin, something so that Connor can get the screws loosened. Jennifer states she can't get her hand in her pocket, she already showed her father her ring, and put it on her finger earlier before crossing the grand lobby across the fire hose, her ring was on her finger, not in her pocket.
When they're climbing up the A/C duct, the water is shown rising in the adjoining room at a decent, but not terribly fast pace. Once the water hits the duct, it seems to magically rise extremely quickly, negating the fact it's still attached to the room and will only rise as fast as it does in the (very large) room.
Computers, electronics, and anything electrical will always short out and not work submerged water. The Bow Thrusters are all controlled by a computer. When Kurt Russell swims through 150 feet of water to turn them off, all of the electronics controlling the thrusters are completely submerged in water. None of these electrical devices should be functioning at all. The Bow Thrusters should be been non operational due to all the computer equipment being under water and shorted out.
Watch the Captain just after he introduces Gloria. There is a shot of him stepping off the stairs at the side of the stage and walking towards the crowd, clapping. They show a closeup of Gloria, then a shot from the balcony. If you look at the side of the stage, the Captain is walking off it again.
When the majority of the passengers are trapped in the now upside down ship after the capsizing in the grand ballroom, the glass dome should now be the floor everyone is walking on - but they're not. The glass dome stays above them. When the dome finally breaks and floods, everyone is looking UP at the impending flood and when the glass shatters, the water cascades in and downward.
Towards the end when everyone is walking towards the bow and they come onto the flooded corridor, the fluorescent lights are all working even under water. The fact is they shouldn't be working at all. The ballasts do not like getting the least bit wet and would be shorting out. The fact that they are working under water, and the ones that are getting constantly getting wet are not arching or sparking is incorrect. The chances of them all getting electrocuted is extremely high. Water and electricity do not match at all.
The ship was traveling the North Atlantic (London to New York) on New Years, meaning the water would be extremely cold. The main stars of the movie would have died from hypothermia, or at the very least they would have been freezing cold and barely able to function.
On a normal right side up cruise ship the pressure relief valves are placed at the bottom of the ballast tanks because of the greater water pressure. But the Poseidon is upside-down so the valve is at the top where the pressure is much less great, so it is impossible for it to open.
After the ship has capsized, every item, including people, plates, tables; are scattered around the room. Gravity would have kept everything to the walls, and when it finally went upside down, most items would have piled up, not evenly spread out.
When the officer of the watch sights the wave, he orders "hard to starboard, starboard engines full astern!" When the capsized ship is shown, it can be seen that the ship (like many, newer, larger cruise ships) lacks a rudder and is equipped with "azipods." These are pods that are external to the hull which contain the motors with the propeller for each motor driven directly. Azipods can rotate 360 degrees and thus eliminate the need for a rudder since the thrust can be applied in any direction.
If an azipod equipped ship needs to perform an absurdly sharp turn, all the azipods would be turned perpendicular to the length of the ship, in the case of Poseidon with the pods facing starboard. This would cause all the thrust to be applied to swinging the ship to starboard. Under normal conditions, this would be very disruptive since it would cause the ship to roll to port and break every dish on the ship. Reversing the "starboard engines," which would be the proper procedure on a conventional ship, would be counter productive with azipods.
It is doubtful the vacuum created by the thrust propellers would be powerful enough to hold the extremely dense compressed gas cylinder against the access door, considering its heavy weight and its aerodynamic shape. It would be like trying to suck up a ball bearing with a vacuum hose. Even if it were strong enough to do it, that would mean it would be far too powerful for Josh Lucas' character to be able to fight, and he would easily get sucked right in.
When they are crossing the upside-down elevator shaft, there is no cable leading to the elevator car which is above them, but there is a cable attached above the car. In the proper orientation, this would be the only lighter-than-air elevator that needs to be tethered down rather than hoisted up.
With the ship flipped over, throughout the entire movie they are walking on the ceilings of the ship. Majority of the ceilings are not designed to hold the weight of one person let alone multiple people. Chances are they would fall through the ceiling multiple times and / or in multiple places. A lot of the ceilings on cruise ships are drop ceilings so when the ship rolled over, a lot of the ceiling tiles would be missing and only guide wires and the sub structure would remain. You don't see any of this in the movie and they don't have any trouble walking on ceilings throughout the movie.
There couldn't have been any working electricity. After the ship capsized, the generators wouldn't have been able to maintain a fuel supply, not to mention generators were falling and the fuel was burning up and exploding. This also means the thrust propellers wouldn't have been able to be running.
At several points in the movie the ballast tanks are shown filling and the ballast gauges are shown going from empty to full. Since ships typically use a large float-type sensor, and the fact that the ship is upside down, the gauges should be reading full when empty and show decreasing level when filling.
In the scenes in the ship's ballast tanks, the characters open the valves to allow water to enter. However, since the ship is upside down, the ballast tanks should not operate normally. Where does the air in the space go?
In the opening sequence where the camera pans around the entire ship, all the lifeboats are facing backward. The propellers are facing towards the bow of the Poseidon. Also sequences involving the Navigation Bridge show the space as being well lit. Ship's bridges are kept in near pitch-black darkness so as not to hinder the navigation watch's night vision. And finally all ship's machinery (like the bow thruster motors) have emergency shut offs in the space with the machinery.
In the sequence at the end of the movie where the ship rolls back over before it sinks, it seems to defy the laws of physics, specifically buoyancy. In a previous shot, it is clear that only the tip of the bow is floating above the water line, representing at most 10%-20% of the mass and volume of the ship, yet when the ship rolls over, at least 25%-30% of the ship is above the water line and buoyant before the ship finally sinks all the way.
The Water seems to chase the fleeing passengers all the way to the bow room. However when outside the ship standing in the bow thruster the water is many meters below. The water level in the ship would be lower than the level outside. The weight of the ship would mean that if such a small volume of air remained the ship would already be under water outside. And when the trap to the bow-thruster was opened the sheer pressure of the air trying to escape would make this one large exhaust - with or without the bow-thrusters.
Screw type propellers designed to work in water, are very poor at moving air, as they are not aerofoil type propellers like the ones used on airplanes. Therefore they would not be able to cause a huge pressure difference in a bow thruster pipe.
According to the original novel and The Poseidon Adventure (1972) on which this film is based, the huge wave was caused by an underwater earthquake. In reality, such waves would be barely, if at all, noticeable by people on a ship in mid-ocean and would certainly not cause the boat to capsize as depicted. Waves caused by undersea quakes increase in size only when they reach shallow areas near the coast, resulting in devastating tsunamis.
Throughout the movie, Richard is seen trying to call his boyfriend, but if he were in fact on a cruise in the middle of the ocean, there would be virtually no cell phone reception. Making it impossible to contact anyone not on the ship.
The bow thrusters have no doors covering them, without them the ships performance would be drastically reduced and the ship would need more fuel, any ship the size of Poseidon would have them to increase efficiency.
At the end when the survivors break out into the open air via the thrusters the effect of this on the "bubble" of air in the ship is not taken into account. This would be exacerbated by the sucking and blowing of the bow thruster propellers (the inaccuracy of which is mentioned elsewhere)
From the time that the ship stabilizes to the time when the ballast tanks start filling the water level outside the ship on the exterior shots does not change significantly whereas the water level within the ship rises the equivalent of 4 to 5 decks.
When the ship's general alarm is sounded. A siren sounds throughout the ship. No passenger ship's general alarm would sound like this. The standard general alarm for passenger ships is seven shorts blasts and followed by one long blast from the ship's horn and an audible tone over the PA system.
They say they are headed to the stern of the ship to escape because the bow is under water, but when they finally get outside of the boat, they exit through a bow thruster port they damaged beforehand (at the front of the ship), not the stern (rear of the ship).
In the sequence where they are crawling up the ducting and Conor unscrews the bolts holding down the grate, there are two or three shots where a hair on the camera lens is clearly visible in the lower left corner of the shot. This was observed in the HD version of the movie, but appeared large and clear enough to show up in SD as well.
When they are in the ventilation shaft, Dylan mentions to Elena about her "little brother". He then asks her if he is older or younger, to which she replies younger. If Dylan refers to him as a little brother it should have been obvious that he was younger than her. Dylan is trying to keep Elena calm. It's possible that he's assumed that her brother is younger in his haste and then realizes he doesn't know for a fact, so he inquires to keep her occupied.
Why would both propellers (at the end of the movie) be sucking air in (or out) at the same time? Bow thrusters would create a vacuum in the chamber no matter which way they are turning, when to Starbord they create a vacuum to suck water and push it to the port side of the ship, and vice versa.
After climbing into a ballast tank, the passengers need to fill the tank in order to open the pressure relief valve that will spill them into the next tank. Aside from the fact that the tank would be impossible to fill (the pump suction is in the air and the tank vent is underwater), and the fact that the pressure relief valve would not open (there will not be enough pressure at the top of the tank), this valve should not exist. A pressure relief valve in a ballast tank allowing water to flow into the adjacent ballast tank would defeat the purpose of having separate ballast tanks. Tanks are kept separate to trim or heel the vessel as necessary, while preventing water from sloshing and de-stabilizing the ship. Having one tank spill over to the next via a pressure relief eliminates that control, effectively creating a "Titanic" scenario where the tanks keep spilling over, forcing the ship to take on an extreme trim and potentially "nose dive" or roll. In the event that a ballast tank is over-filled, the water would simply spill out the tank vent.
The film's ending implies only the 6 remaining characters made it out alive, it seems highly unlikely that only 10 people (with only 6 surviving) on a boat with over 400 passengers would actually attempt to escape the sinking ship, surely there should have been more survivors.
Cruise ship poker dealers are certified professionals, however, the Poseidon dealer allows both "splashing the pot" (thowing chips directly into the pot) and a "string bet" (placing chips into the pot as a call and then placing more chips in as a raise). These are basic rules violations of professional dealing and would never be allowed in a cruise ship casino, especially in a high-stakes game such as the one in this movie.
During the capsizing, there is a shot of the an indoor pool with a wave proceeding from the raised end down it's length. While dramatic, this wave cannot occur, as all water rises at the bottom end of tilting vessels.
When Josh Lucas pushes the flammable canister into the Bow Thrusters to disable them, the explosion and shrapnel from this container would have impaled and/or severely burned or killed him from the explosion it created. He could have not survived his body being sucked into the tube and then live through fire and metal shrapnel being flung everywhere.