The way out is on the other side of the large 'hangar' that you're swimming around in. In the upper northeast corner (using the map-view) there is an inset fan/ventilation shaft panel in the wall. [it's set in the eastern wall, two panels down from the ceiling and three panels to the right of the northern wall.
Open it like any door and there will be a new set of passages to explore. Go through it, swim up, start heading down the passage, but make sure to hang a left at the first junction you come to. After that, if you keep an eye on the ceiling you'll soon see another shaft you can swim up--do that and there's a pattern buffer waiting for you.
(farther on you can find an alternate way into that room with the faulty blast door...the ventilation passages are the key to getting everywhere here)
