People are playing water volleyball.
People are watching a volleyball game.
The fact that people are playing water volleyball doesn't imply that there are people watching their playing and it is a volleyball game that is going on.
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_play(e1) & n_volleyball(x2) & of(x2,x3) & n_water(x3) & n_people(x1))
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_watch(e1) & n_game(x2) & of(x2,x3) & n_volleyball(x3) & n_people(x1))