I've seen a lot of fans talk about the scene where Katara runs out of water in the new movie and are upset that she isn't doing anything like taking water out of thin air, using her sweat, or bloodbending. And there are three things that they are forgetting here. One, yes, Katara and Hama could take water out of thin air, but as the show displays to us through the use of visual language, there is a limited amount of water that Hama is able to take from the air itself. She's only able to take just about enough to fill her fingers, which she then freezes into ice to use as projectiles. And for those of you who may think, well, they don't explicitly say that she can't take more. True, they don't through the use of dialogue. However, if Hama could take more, then she wouldn't have needed to use or show Katara the trick with taking water out of the flowers, which visually shows us having more water than the water she was able to bring out of the air. If Hama could reliably take water from the air to use as if it was already around her, she wouldn't have needed to use the flowers at all. Second, yes, Katara can use her sweat to bend, but just like the water from the air, it is a smaller quantity and requires time for buildup. She doesn’t use the sweat in a big blast, but instead uses to slice and whittle down the wooden prison.
And third, and this is very, very important to remember when it comes to waterbenders, most waterbenders, even Katara, needs the use of a full moon in order to bloodbend. Both times that Katara bloodbended was during a full moon. And there is no indicator that she ever learned how to get past this limitation in the future based not only on the fact that she doesn't want to bloodbend, but also because of what we know in Korra. In Legend of Korra, Yakone, a bloodbender, tries to use the known fact that bloodbending cannot be done unless on a full moon as a defense. Of course, as Sokka points out, he has met all kinds of strange benders, using Sparky Sparky Boom Man as an example and that is feasible that Yakone could be a bloodbender without the full moon, but he never references his sister or any other waterbender of being capable of this feat.
Could Katara have managed something offensively? Sure, but their backs were against the wall and surrounded by airbenders, and her goal at the time was defense from multiple attackers.
