Preheat oven to 350 degrees F. Combine cake mix, water, 1/2 of the condensed milk, oil and eggs in mixer bowl; beat well. Spread half the batter into greased 9x13-inch cake pan. Bake for 30 minutes. Melt caramels with remaining condensed milk in saucepan, stirring frequently. Stir in pecans. Pour over hot cake. Spread remaining batter over caramel layer. Bake for 20 minutes longer. Spread White Turtle Frosting over warm cake.
For frosting: Combine margarine, confectioners' sugar and milk in mixer bowl; beat well. Add vanilla; mix well.