Zuko has to bite back his instinctive, How would you know? This pond hasn't even existed for more than a month, and instead modifies it into a more diplomatic reaction. He's learning. Slowly, incredibly slowly, but it is happening.

"Have you been here long?" he asks, conscientious of how recognizable he is even out of uniform, so to speak, and striving for composure. He realizes he hasn't actually answered the implied question and tacks on belatedly, "I forgot how claustrophobic you can feel after being on a ship for that long." Hence why he's out here.