Prepare gelatin according to package directions. Meanwhile, preheat oven to 350 degrees F. Grease and lightly flour a 9x9-inch square baking pan; set aside. Prepare cake mix according to package directions. Pour batter into prepared pan. Bake in preheated oven for 25 to 30 minutes or until a toothpick inserted near the center comes out clean. Cool cake in pan on a wire rack for 10 minutes. Remove cake from pan; cool completely on a wire rack. Cut a 4x4-inch square 1-inch deep in the center top of the cake. Remove cake inside the cut square, being careful to only remove cake to a depth of 1 inch.
Place half of the frosting in a small bowl; tint with yellow food coloring to desired shade of yellow. Tint remaining frosting in the can with blue food coloring to desired shade of blue. Place cake on a large serving plate. Frost the sides of the cake with yellow frosting. Frost the top of cake and insides of "pool" with blue frosting.
Decorate the sides of the cake with jelly beans and fish-shape candies. Decorate the edges of the cutout pool with small green and yellow candies. To make a diving board, cut one sugar wafers in half crosswise. Attach half of the wafer to the candies on the edge of the pool using some of the frosting. Cut another wafer to make a 2-inch-long piece. Use fristing to attach the end of the 2-inch wafer to the top of the other wafer cookie, to make a T-shape.
Up to 1 hour before serving, stir the gelatin to break up. Just before serving the cake, spoon some of the gelatin into the center of the cake (reserve any leftover gelatin for another use). Arrange some ring-shape jelly candies in the gelatin. If desired, place tall, thin candles into the cake as patio torches and/or place a few cocktail umbrellas into the cake. Makes 12 servings.