Percy Jackson is Harry Potter and Sally's son. Somehow, Percy ends up at Camp Half-Blood and most of the people think he is Poseidon's son because of his looks and most of them saw him conjuring water. He vehemently denies it but everyone is determined to make him reveal the 'truth'.
Zeus and Hades are accusing Poseidon of breaking the oath and he is just tired of denying it.
Harry is laughing his ass off seeing all the chaos.