Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch pan.
In a large bowl, stir together cake mix, egg whites, oil, water and margarita mix. Pour batter into prepared pan. Bake in the preheated oven for 25 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.
Stir lime zest into the Cool Whip(R). Frost cooled cake and refrigerate until serving.
