Nancy quickly realize that they were fully empty of air now, and Max is struggling for air already. Even then, she decides to share her air with Max, at that moment, she only cared on keeping her safe.
Lucky for the two of them, the intruders decided to leave the pool, loud and clear enough where the two of them could hear it.
The way to the surface was long, but they eventually made back to take a sweet breath of air...and probably leave the pool for the night.
Well...at least after they retrieve the tanks at the bottom.