Actually, the arsenic atom gained 2 electrons. Its oxidation state went from 5+ to 3+. The nitrogen, on the other hand, lost 2 electrons, going from 3+ to 5+.
As for H+ & H2O, look at the number of oxygens & total charge on each side of your equation. It's not quite balanced, though you can balance this if you include the H+ & H2O.