Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, combine the cake mix, water, vegetable oil, and eggs. Beat on medium speed for 2 minutes until well combined.
Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Once the cake is done, remove it from the oven and let it cool in the pan for 10 minutes.
Using the handle of a wooden spoon or a fork, poke holes all over the top of the cake, about 1 inch apart.
In a separate bowl, dissolve the strawberry gelatin in 1 cup of boiling water, then stir in 1 cup of cold water.
Pour half of the strawberry gelatin mixture evenly over the cake, allowing it to soak into the holes.
Repeat this process with the blue raspberry gelatin, using the remaining half of the mixture.
Refrigerate the cake for at least 2 hours to allow the gelatin to set.
Once chilled, spread the whipped topping over the cake and decorate with sliced strawberries and blueberries.