Daine recognizes Peeta from a fair distance - not with her eyes, but with her echolocation (which, in this shape, is even better than a bat's). She considers trying to sneak up on him, but then realizes he probably hasn't seen a dolphin before. Best not to frighten him just for a joke.

So she surfaces a few yards from his boat, coasting on her side so she can examine him with one eye. She keeps her mouth shut (it looks as if she's smiling, and he can't see how many sharp little teeth she has). Then she lifts her pectoral fin and waggles it in a little wave.