The realization that he isn't alone does startle him; it's the sight of the second stone just before it crashes into the surf that makes him falter with the rock in his hand, turn on his heel, and look towards the source. Who could have thrown that?

He doesn't recognize the stranger, and there's just... something about him, but to avoid being rude, he lifts a hand in greeting anyway, offers a small smile. He'd say hello, but the surf is just loud enough that at this distance, there isn't really any point.