Preheat oven to 350°F. Prepare cake batter according to package directions. Pour 2/3 cup batter into a small oven-safe bowl and the remainder into two 8-inch round cake pans. Bake according to directions; let cool completely.
Meanwhile, place 3 Tbs. frosting in a small bowl and tint dark brown with food coloring. Spoon into a small ziptop bag and cut a small snip from the corner. Repeat to make 3 Tbs. each of blue, green, and orange frosting. Using matching frosting, attach two Starbursts to form a square hut and top with two shredded-wheat cereal squares to form a roof. Pipe a doorway and a line where the cereal squares meet. Lay Teddy Grahams face-up on a flat surface. Use the dark-brown frosting to pipe on facial features and the other frosting colors to pipe on bathing suits, leis, and flowers in hair.
Cut sour green-apple candy straws in half crosswise. Use a paring knife or scissors to make several slits 3/4 of the way down on the halved candy straw. Insert the solid end into a pirouette cookie and use leftover green frosting to attach Cocoa Puffs for coconuts, as shown.
Tint 1/3 cup frosting light brown and tint the remaining frosting light blue. Frost the small bowl cake with the light brown icing. Grind shortbread cookies in a blender or a food processor and sprinkle over the light-brown icing while still wet. Frost the top of one 8-inch round cake with light-blue icing, then top it with the second 8-inch round cake and continue frosting the sides only with light blue icing. Use a spatula to position the bowl cake on top of the stacked cake. Finish icing the top of the stacked cakes with light-blue icing, overlapping blue icing up with the sides of the center island to create a subtle wave effect.
Use scissors to trim Fruit Stripe gum into a surfboard shape and place along with huts, palm trees, and the Teddy Grahams in position on the top of the cake; secure with frosting if necessary. Cut the Twizzlers on an angle at various lengths and place on sides of cake for seaweed while the icing is still wet. Finish decorating the base of the cake with Nerds and jelly beans for ocean rocks and add Swedish Fish and remaining candies as ocean floats.