Preheat the oven to 350 degrees F. Line a half-size sheet pan (18 x 13 inches) with parchment paper.
Tip the cake mix into a large bowl. Add the eggs, vegetable oil and water (in the quantities given on the box of the cake mix). Stir together until smooth. Mix in enough blue food colouring to get a vibrant blue colour.
Pour the batter into the prepared pan, spread out into an even layer and bake for 15-20 minutes until a skewer inserted into the centre of the cake comes out clean.
Once baked, set the cake aside on a wire rack to cool completely.
Prep the ice cream:
Let the vanilla ice cream soften for a few minutes then scoop it into a large bowl. Beat with a wooden spoon to soften to a buttercream-like texture. Crush the cookies into rough chunks and add to the bowl of ice cream with the rainbow sprinkles. Stir together until combined.
Layer the cake:
Cut three rectangles of cake using a rectangular (10 3/4 x 5 1/2-inch) ice cream cake pan as a guide/template.
Line the ice cream cake pan with parchment paper.
Place the first layer of cake into the bottom of the ice cream cake pan.
Spread half of the ice cream mixture over this in an even layer.
Top with a second layer of cake followed by an even layer of the remaining ice cream mixture. Finally, top with the third layer of cake. Cover with a layer of parchment paper and.
freeze for at least 5 hours, or overnight, until solid.
For the blue frosting:
Once the cake has frozen sufficiently, prep the frosting. Pour the heavy cream and sugar into a large bowl. Add enough blue food colouring to make a vibrant blue colour.
Use a whisk to beat the heavy cream until thickened to soft peaks.
Remove the frozen cake from the cake pan and turn out onto a serving plate. Remove the parchment paper.
Cover the top and sides with the whipped cream, making swooshing wave patterns on the top using a small spatula or butter knife.
Decorate with the finger wafers around the edge of the cake. Place a few gummy rings on top with graham cracker bears inside them. Add a few candy balls to resemble beach balls and add a few more graham cracker bears to the top of the cake like they’re swimming.
Leftovers can be kept frozen, covered for a week more (but keep in mind that the cookie bears on top will soften once you defrost it again).
Buy the Pampered Chef pan here: https://www.pamperedchef.com/shop/Bakeware/Silicone/Ice+Cream+Cake+Pan/1551
OR use a silicone loaf pan (10.5 x 6 x 2.5 inch). Just line with parchment paper leaving some overhang at the edges so you can lift the cake out of it once frozen.