Beat together margarine; peanut butter and sugar until creamy. Add water, egg and vanilla; beat well.Add combined oats, flour and baking soda; mix well. Cover; chill about 1 hour.
Heat oven to 350 F.Shape dough into 1-inch balls. Place on ungreased cookie sheet; flatten with tines of fork dipped in granulated sugar to form crisscross pattern. Bake 9 to 10 minutes or until edges are golden brown. Cool 2 minutes on cookie sheet; remove to wire rack. Cool completely. Store tightly covered.