A lot of what this fic does with Koh confuses me, tbh. His dominion over the souls of the unhallowed dead is another example. But, yeah - he's a giant centipede who lives in a tree and steals the faces of people an animals who show emotion in his presence. None of that has any connection to water, not much of one to death (losing your face could count as losing your soul or identity, which might be a form of death... but we see a monkey whose face Koh stole which was still alive...) or, spoilers, any indication of being the Avatar Spirit's child. *shrugs*