Damon might have dodged that punch if he hadn't been staring at the ghost at the other side of the railing. He never exactly made it to that time in Mystic Falls where all the dead people started appearing so he isn't used to seeing ghosts. However, he was there when Anna died. He watched as John killed her.

He's about to say something about the fact when Ethan punches him in the jaw and then tosses him back on to the deck.

Damon grimaces, shooting daggers back at the werewolf before he looks up at Anna from his spot on the deck, sliding a hand over his jaw. "...I thought you were dead."