We can see that all of Aang's meaningful moments happen in water-related settings, like when he originally runs away from the Southern Air Temple and when he 'runs away' from the ship at the beginning of S3 after loosing to Azula, and I'm pretty sure there's one or two more (though I could be wrong) which kind of has me wondering why Aang doesn't have more difficulty with waterbending. I would expect Aang to have flashbacks and a mental block when he's learning waterbending as he's overcome with his trauma from running away.
Despite that, I do realize that Aang's fighting style and affinity for airbending and airbending techniques makes earthbending the obvious element for him to have trouble with, I still think it would be interesting if Aang had at least some trouble waterbending after all of the difficulties that he's faced in the water.
