Melia grinned at the surprise she could read in the man's body language; she watched him stick his face in the water and look around, presumably for whatever had touched his foot. She'd immediately dived deeper, out of sight in the darkness of the deeper water, and watched him until he needed to go up for air. When he did, she shot upward, wrapping a supernaturally strong hand around his ankle and yanking downwards. It was just play to her; she certainly wasn't holding him down, and had let go as soon as his head went under. If she'd intended to do real harm, she wouldn't have let go. Drowning humans was certainly one of the best ways to kill them because it didn't bruise the meat the way other methods could, but she didn't want to drown him.
Yet.
Just after letting go of the human's ankle, Melia shot upwards, behind his back, and slid her hand teasingly through his hair before flitting away again into darker waters - though not so dark as before. If he was observant, he might get a glimpse of her. It would be interesting to see whether or not he'd chase her. Some humans fled screaming, but others - usually the ones who ended up dead - let curiosity get the better of them and gave chase.