It wasn't by skill that Beleth avoided hitting the bottom like Norrington had. In fact, it was mostly Norrington that let her avoid it--she hit into him, but he wasn't quite as unforgiving to smack into as the rocks, and Beleth managed to disentangle herself, swimming away from him so they could both surface.

But after a few moments of struggling against the instinctive panic bubbling up, she puzzles out which way is up and kicks to the surface.

Her reintroduction to air is met with a couple of wheezing coughs, and then sputtering, as her attempts to tread water wrestle with her trying to cough. She relaxes a little when she sees that Norrington has popped up as well, and manages to cough out a "Oh good, you're not dead."