Kija being very capable of climbing was also something he knew of, if basing it off of the dream-self Jae-Ha witnessed.
"Well I've done so numerous times before, haven't I?" Hak was far heavier than Kija, anyway. Shin-Ah as well. "I really don't mind it at all."
He can't imagine that the water huts really were unstable, especially seeing how even a massive ship could (somehow??) stay afloat, but the tree was all at once more secure. Despite how much Jae-Ha does enjoy being on the water, and knew very well how to swim, he can't counter any mishaps with his ability.
"Nope. Unless Shin-Ah-kun suddenly needs us or anything." Because he can only imagine that if Shin-Ah did, he'd use his ability to sense them, rather than just utilize the communicator. Like anyone else.