Still holding the ball, Seamus hadn't been able to duck in time and so Susan's wave of water hit him square in the chest. As he was already drenched, he just laughed and made a show of 'hiding' behind the ball. "Can't get me!" he taunted, then immediately launched the ball towards Dean, flying far over Susan's head. Susan and Dean might have the advantage of being fitter, but Dean and Seamus had been playing football and quidditch and variations of catch together for years. They had the advantage of knowing one another's moves.