If you cast Air Walk underwater (or submerged in another liquid), is the subject borne toward the surface just as if you had cast Water Walk? Air Walk doesn't say so explicitly, but I wonder if the two spells would/should act similarly in such an instance.
Air walk, explicitly says you can tread on "air" as if it were ground. So, I'd say it has no effect while you are underwater. You could treat the air as a shore, to step out of water if you are already on the surface though.