Aang didn't exactly master water in that episode though, he did use the water but he only really used basic moves to attack using water and Toph I guess was teaching Aang how to Earth bend inbetween episodes, her pushy-ness must of gotten Aang to learn earth bending faster then water bending which is why Aang seemed to progress his earthbending abilities a lot in a short amount of time. The element Aang used most in the battle was earth so maybe he was trying to see what he could do so far with the earthbending he already knows. Sorta wierd though, the only element Aang really ever used in the other episodes beside this one(and 'Return to Omashu') is air but air is the element he uses least in this episode