[Woah, wait, what? Arthur did not sign on to go take this mission with a child. He wouldn't have probably let his own son come along at that age, and he isn't sure he wants to be responsible for someone else's kid. He glances back over his shoulder toward the person he thought he was talking to, who seems to be gone, and then back to the kid that answered.]

I'm not sure you're old enough to do something like this. I'm sure you're very talented to be here in the first place, but you won't be able to keep up with me. [He says it matter-of-factly, non-judgmentally. No one, save maybe a Flash or Superman (or another Atlantean), could keep up with him in the water.] Especially in the dark.