A young man throws something in the water.
The man is littering.
Fish food may be thrown in water, so anything  that is thrown in water is not littering.
exists e1 s1 x1 x2 x3.(n_water(x3) & Theme(e1,x2) & Actor(e1,x1) & v_throw(e1) & in(x2,x3) & n_thing(x2) & n_man(x1) & a_young(s1) & Theme(s1,x1))
exists e1 x1.(n_man(x1) & Actor(e1,x1) & v_litter(e1))