Working in 2 batches, blend watermelon and lemon juice in processor or blender until smooth. Transfer to pitcher.
Bring 3 ½ cups water and sugar to boil in medium saucepan over high heat, stirring until sugar dissolves. Add hot syrup to pitcher with watermelon-lemon juice. Refrigerate until cold, about 2 hours. Fill tall glass with ice. Pour Watermelon Lemonade over and garnish glasses with lemon wedges.