Kisame watches impassively as Haku absorbs the boon--and boon it is--and gives response. "And how would you prefer to return that favor, kid?"

It's not so much the repayment, as the security of knowing he can force an ally from the most unexpected corner if he needs to that Kisame is after, so he's not interested in balancing this favor any time soon. But, if the kid offers something attractive, Kisame is willing to consider letting it be tallied up and finished. So he waits, patient and assessing.

Perhaps the kid knows the clan secrets? The chakra is a sign in that direction, but not an ironclad proof. Kisame doesn't know how much time they would take to learn, or even if the kid had a chance to learn from someone older. Well, he'll find out soon.