Factual error: When performing CPR on Elena, Robert doesn't tilt her head back. As a former firefighter, he would know one of the first steps of CPR is to tilt the victim's head back to clear the airway. His rhythm is also way off.
Suggested correction: Tilting the head back to clear the airway isn't practised any more, due to exacerbating possible neck/spinal injuries. Instead, one would do a chin lift to clear the airway. But alas, he didn't do that either.
Paramedic here. The head tilt-chin lift is absolutely still practised today and is definitely our go to manoeuvre for CPR. You're thinking of the jaw thrust manoeuvre where we do it when we suspect spinal injury, which Elena did not have.
Former Volunteer First Responder and Ambulance driver part-time but also volunteer here. It actually depends on the SOP of the company in which you serve. Liability purpose has us practising caution in otherwise duty-to-act scenarios. While you are always to remain either Red Cross or American Heart certified in CPR, ALS guidelines and SOPs still govern the practice in which you are performing and can be effected by the decisions of your local Medical Director, but I digress.
Factual error: In the scene where everyone is in the first ballast tank and the water is turned on, there is air inside the ballast. As the water level rises, the air will become more and more compressed at the top until eventually there should be a thin cushion of air on the top with sufficient pressure to resist any additional flooding. Now I know nothing about the construction of ballast tanks, so there may be a vent on the top to let air out as it fills normally. If there is such a vent, it would now be in the bottom since the ship capsized. So when the characters rise up to the top, that is actually the bottom of the tank with no holes/vents for the air to escape.
Suggested correction: It's right that during filling a thin cushion of air forms in the top of the tank, and in a normal situation the air would be vented from the tank. But keep in mind the water rushes in by "gravity", which is more or less open a valve and let the sea do the work. That goes with a ton of hydrostatic pressure that no amount of pressurized air will stop.
Factual error: When the ballast tank valves open, they open by dropping down, which would be impossible with the pressure of the water pressing against it. Regardless of the domino effect of the water flowing, it's tremendous pressure.
Suggested correction: It's actually very much possible. These PRV are designed to open when the pressure in the tank reaches a certain setting, which is equal to or slightly higher than the max design pressure of the tank. Plus, a gate valve of that size can only be operated with hydraulic oil plungers, capable of generating 150+ bar to lift the valve in a normal operation. Since the ship is upside down, the valve slides down; the weight of the valve then would require less hydraulic pressure to open.
Factual error: After the first ballast tank is flooded, the others are shown filling during the rest of the movie. But since the ship is upside down and the valves therefore at the top of the tank, all of the water is below the valve, and couldn't possibly flow through it to the next tank.
Suggested correction: I believe the actors appeared in a side ballast tank, judging from the slight curve at what used to be the bottom. It is then viable that the entire tank was below the waterline; the way they opened the valve made the tank directly subject to the sea. If the tank was entirely below the waterline it would fill up completely, even with the valve open, spilling water into adjacent tanks in a cascade effect.
Continuity mistake: When the survivors are climbing out through the propeller tube, Dylan is the first one out with the others following him. But after everyone is out he is the last out to climb out.
Suggested correction: It's very obvious that Dylan is not getting out, but merely looks inside the hole, and is then standing up, so he was there all the time after he got out first, and then just looked into the hole, after everyone got in the tube, then gets up.
Factual error: The ballast tanks would not open once they are filled with water. They are designed to hold the water in that part of the ship, just in case one fills with water, the whole ship will not sink.
Suggested correction: The pressure relief valves in the Poseidon's ballast tanks are there as a secondary safety device against excessive pressure due to overfilling. The primary safety is a tank vent on the first weather tight deck; it can happen such a vent gets clogged or frozen, therefore not allowing the pressure that during filling builds in the tank to escape. The PRV will have a setting slightly higher than the max design pressure of the ballast tank.
Other mistake: When the ship first turns upside down, power is lost throughout the ship and emergency lighting takes over. Yet shortly after, people are being severely electrocuted in the Disco.
Suggested correction: The lights went out because the power cables connecting them to their power supply were severed. Just like a downed power line in a town without power, there is still an electric current traveling through these lines, but the lights go out because they are no longer connected to that power supply.
That's not true. The ship, when upside down, suffered a blackout, since the engines that drive the generators cannot run upside down. The only power source available then are the ship's batteries, which certainly can't and don't provide the amount of voltage that apparently still coursed through the line in the disco. It should've been dead, like everything else on the ship not connected to an emergency power source.
