Kingsman

Training scene where they shove a shower hose down a toilet and use it to breathe...

There would be no air (or even sewer gas) to breath in that case. Toilets work by raising the water level in the bowl above the water level in the S-bend/siphon. Since the room was full of water, those toilets would have been flushing constantly, and the whole pipe would be full of water.

Better(ish) solution. Use the body bags that they each had to fill out and place in their trunk/locker to capture an air bubble. That would at least give you some time to attack the door, or figure out how to drain the room.