Can you really breathe through a submerged toilet?

For a complete answer, see this identical question on movies stack exchange. However, the brief answer is: P-Traps and vent pipes.

If the water from the room were to make the toilet act as a drain, there would still be air from the vent pipe coming from above, as the water would flow downwards. It would be a nightmare trying to get the tube into the correct position, but possible.