If you could get the breathing tube to the vent pipe part of the plumbing there might be air in that part of the piping even while the toilet is draining the wa

If you could get the breathing tube to the vent pipe part of the plumbing there might be air in that part of the piping even while the toilet is draining the water from the flooded room. You would need to get the tube pushed past the toilet and into the main drain pipe though to have a chance at hitting the vent which would be difficult. I would be concerted about the gases that come out of a waste pipe though in either case I would think that can't be good for you.