I can deal with Amon waterbending to save his life, but I still think the way he did it was out of character for him. Using waterbending to get to the surface? Sure, that makes sense, but emerging from the water and floating in midair on top of a giant water column for such a long time? That was too much.
As suggested above, it was probably reflex. He had been unconscious, he woke up in the water, and he reflexively retreated from that situation as forcefully as he could. And he seemed to be up there longer than he was because of the slow motion.