Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch baking dish.
In a large bowl, whisk together the cake mix, water, oil, and eggs until smooth. Pour the batter into the prepared baking dish.
Bake for 23-28 minutes, or until a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the pudding. In a medium bowl, whisk together the pudding mix and milk until thickened.
Once the cake is done baking, let it cool for a few minutes. Then, use the handle of a wooden spoon to poke holes all over the top of the cake.
Slowly pour the pudding over the cake, making sure to get it into all the holes.
Spread the whipped topping over the pudding layer.
Sprinkle the vanilla wafer crumbs over the top of the whipped topping.
If desired, slice the lemon and arrange the slices on top of the cake.
Refrigerate for at least 2 hours before serving.