Preheat oven to 350 degrees Fahrenheit. Spray a large pyrex pan. In a stand mixer with a large bowl, combine the cake mix, water, eggs and coconut oil. Mix. You might have to split up the boxes of cake because it might not fit in the stand mixer.
Pour batter into the large pyrex.
Bake for 25 minutes, until golden brown. Let cool five minutes.
In a saucepan, combine 3 cups blueberries and 1 inch squarish chunks of watermelon, sugar, zest of one lemon, juice of one lemon, 1 1/4 cups water and 2 tbsp cornstarch.
Bring the mixture to a boil over medium heat. Simmer for 5 minutes, stirring constantly.
Using the handle of a wooden spoon, poke the cake every inch in a line.
Spoon blueberry and watermelon sauce evenly over the cake.
Chill the cake for one hour.
In a stand mixer, beat 4 cups heavy whipping cream, 4 teaspoons vanilla extract, and 1/2 cup powdered sugar until soft peaks form.
Top cake with whipped cream and fresh berries and watermelon. Enjoy!