To prepare the jello pool, add the apple and blue spirulina to a pot and whisk well. Sprinkle the gelatin over the juice mixture and lightly mix. Allow to harden for 2 minutes.
Transfer the pot to the stove and heat over low heat for 3-4 minutes or until fully liquid.
Pour the liquid into a 2 cup glass container (you can also use a silicone container for easier removal) and transfer to the fridge for at least 3-4 hours or until fully set.
Prepare the cake as instructed in 9" cake pan. Allow to cool fully before frosting on all sides.
Carefully remove the jello pool from the glass container and place it in the center of the cake.
Decorate the pool with fruit leather to make a barrier around the pool, add gummy rings with gummy bears, and unreal gems for beach balls. Finish the cake with paper umbrellas.
Carefully slice and serve chilled! Store in the fridge for a few hours if making ahead of time.