Two young boys are playing in the water.
two twins play on a beach
Water can be found many places, not necessarily a beach. The two boys could have many relationships (friends, older or younger, etc.) so you cannot infer they are twins.
exists e1 s1 x1 x2.(n_water(x2) & in(e1,x2) & Actor(e1,x1) & v_play(e1) & n_boy(x1) & a_young(s1) & Theme(s1,x1) & card_eq_2(x1))
exists e1 x1 x2.(on(e1,x2) & n_beach(x2) & Actor(e1,x1) & v_play(e1) & n_twin(x1) & card_eq_2(x1))