Poseidon/Headscratchers

At least one character is shown carrying a bottle of water. This suggests that such bottles are attainable on this ship, as they are just about everywhere in the world, but at the very least we know they have one. Now, what happens to a bottle of water when you empty it? it becomes a bottle of air. I don't know about most people, but if I needed to keep swimming through tunnels and whatnot, the first thing on my mind would be "containers of oxygen, for the love of God, containers of oxygen". So why doesn't this thought seem to occur to any of the characters in this movie?
- Okay, you have a bottle of oxygen. What the hell are you going to do with it? Swim along the tunnel, open the bottle, and breathe from it? Tape a pipe from the bottle neck and put it in your mouth? A bottle of air is pretty useless unless you know how to MacGyver it into a useful breathing apparatus, which I imagine no one had the knowledge, time, skill, or calm in the face of panic to do so.
  - Plus, even if they managed tp stick the neck of the bottle into their mouths and properly breathed through it under water...In time, they would be breathing in CARBON DIOXIDE!!! Y'know, the thing the lungs politely asks to leave every time you exhale?
  - It's not like they're on a three-hour tour of the flooded hallways-they end up only having to go as far as they can hold their breaths, and having something to breathe into would extend their range and possibly help them find safer routes. The "burning lungs" feeling when you hold your breath isn't from lack of oxygen, it's from the bit of extra carbon dioxide. The body actually can't detect how much oxygen it contains, only carbon dioxide, so it screams at you to exhale while there's still plenty of usable oxygen in your lungs. It's theoretically possible to sustain an adult on one breath for one hour if they breathe pure oxygen, but they'd still be able to hold their breath for only about 4-5 minutes because the carbon dioxide buildup would override their will and cause them to involuntarily start breathing again. A bottle of air could be useful in delaying or preventing that reaction and extending how far you can swim. And who needs MacGyver? Hold the bottle in your mouth, secured with one hand like you used to do at the swimming pool.
- Next time you're at the pool why don't you fill a bottle with air and then go under and breathe with it.
Did no one else on the ship survive? No one else managed to get off the ship? No one else was alive in all the compartments they went through? With that many people, it's kind of hard to believe.
- It was a sudden, shocking incident, and the water was pouring in soon after.
If they were all such great swimmers, why didn't they just find their way to a stateroom and break the window, then swim to the surface? They probably would have spent less time under water, total.
- Perhaps they didn't think the windows were breakable. Keep in mind that the captain (who remained in the ballroom) clearly did not think that was going to happen, even though he turned out to be wrong. Also, as you can see from the ballroom scene, the water would have come rushing in very rapidly, flooding the room, probably drowning them before they had a chance to get out.
- Okay, they decide to break a window open. How are they going to climb through with a large amount of ocean water pouring in?