Hmmm... well, John would make a show of not wanting to let her play due to her tender years (it doesn't much matter to him, but he knows he's "supposed" to do things like that). So all she would have to do is pitch some kind of a fit about "I do know what I'm doing and I'm not a baby and I'm smarter than you so there nyah", and John would shrug and proceed to school her soundly in just why little girls should not dive into the water with sharks. He would not hurt her in any way -- he *hates* people who hurt or abuse children -- but he'd make sure she learned a valuable lesson.