Why didn't Katara think of using such technique (which she did in The Swamp episode) before coming to the eureka moment "my sweat can be waterbent!" ? I mean unless I'm grasping the concept of the ability works, but she doesn't really need to be concern whether she's deprived of bendable when she could have breath a mist of vapor out of her lungs through her throat to freeze the wooden prison and thus shatter it or rebend it into water then slice down through the barrier.