Preheat oven to 350°F. Place apples upright in oven-proof baking dish. In a small bowl, mix raisins, walnuts and cinnamon together. Fill each apple with 2 tablespoons of the raisin mixture.
Mix maple syrup with water and pour over the apples. Bake 30 minutes or until apples are tender. Occasionally baste with maple syrup sauce. Serve warm with a dollop of vanilla yogurt if desired.