I thought that, too, but I just rewatched it, and the door definitely swung out into the main moonpool room...I'm guessing it is designed like that so that if the moonpool floods, it is easier to lock yourself in the comm room to be safe. (the water pressure from the moonpool room would help close the door for you)

So, once the comm room started flooding, you could never close the comm room door...you'd be pushing against the flow of water. I assume Charlie figured he'd close himself in there before the water started flooding out into the moonpool room.

However, since both of them were able to swim down from the surface in time without drowning, I don't know why they even cared about keeping the water from flooding the whole thing...once the flooding started, just run out of the comm room, grab Des, jump in the moonpool, and they both could swim up to the surface. Who cares about the rest of the station...just get out of there!