The door could only be locked from inside, so would not have sealed you idiot, then the whole place would have flooded!
but... they were leaving! there was nothing left to do in the station, they could have just left.
plus, even if the door was wide open, it's not like the
entire station would have filled with water in seconds. it was huge! they could have hung out for a half hour or so, saying how much it sucked that water was flooding in, before deciding to swim back to the top. ...through more water.
Actually, no.
You might recall that there was a pool/entry into the station from underneath that was open to the ocean which people could use to swim up and into the station. The only thing that keep that pool "level" and prevented it from filling up the station with water was the air trapped in station.
If Charlie hadn't kept that door locked/shut once the water started coming in the porthole, the "air bubble" would have collapsed and water would have started rushing in and up through that giant swimming pool sized opening the bottom. Most likely at that point the whole station, with Desmond in it, would have sunk like a rock.
In addition, I think a valid argument can be made that Charlie "wanted" to die. He had cheated death many times (if you believe Dexter's premonitions) and the more he did so the more he was convinced that had eventually had to die to save Claire and the kid. Therefore, even if Charlie could have found a way out in time, I don't think he wanted to.
