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In Poseidon, in one scene people flooded themselves by opening a nearby valve. Water filled the space and a hatch also got opened which lead to the next room.

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Luckily they moved to the next room but they did so by risking themselves. Actually the ship is sinking and the next room may be filled with water or not. How did they know what is on the other side?

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How did they know what is on the other side?

They didn't...

..but they made a reasonable guess based on the information available of where they were.

Plus...after all, they had nowhere else to go!

IMDB

They end up in some kind of ballast chamber, and Dylan realizes the only way out of it is to flood the chamber so the vent on one side of the room opens. They let the water in and after a while, the vent opens and they swim for some stairs.

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