I don't know if I'd equate that with the waterbender's Swim ability. To me, it seems like he used the momentum he built up while flying combined with (perhaps) the use of the Propel seed. While I'll grant that he definitely moved faster through the water than a normal person could, I don't think it justifies a class ability. Again, if the consensus is to keep the Swim ability, I'll gladly go along with the group, but I just don't see the show justifying it.
We already have a seed that mimics that ability. It's a part of the Earthen Stride earthbending seed.
Hmm. I don't know how I feel about this one. On the one hand, ties between firebending and the breath are explicitly mentioned in the show (The Deserter, Bitter Work, etc), and this could be a good way of representing that. It could also give firebenders a boost to damage that's useful but not out of hand. One the other, this specific ability is not, and I'm not sure whether we really need to represent this aspect mechanically. Breath control could just be a part of being a firebender. I guess my jury's still out on this one, though I'm leaning in favor at the moment. I'll wait and see what others think.
Justifying this feat by its ability to power up multiclass firebenders is interesting, but I think I'd support a Practiced Bending feat that increased bending level like Practiced Spellcaster instead of this sort of solution. Again, I'd like to see what others think.
Mephibosheth