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With airbending, you gather the air from the surface and you keep bending it around you to push the water aside while your going through the ocean. Pretty much how you make a pocket of air in your drink by blowing air through the straw. Does that clarify for you?
It may not be exactly cannon but we know that air does push things away so there is no reason not to believe that a constant flow of air can keep water away. AKA Air bubble.