Make fish toppers. Melt red, orange and yellow Candy Melts candy, separately, according to package directions as needed. Place fish cutter from set on non-stick cookie sheet. Fill cutter with melted red candy. Tap to smooth. Chill until firm, about 10 to 15 minutes. Unmold. Repeat to make one orange and one yellow fish.

Make sea plants. Melt dark green, vibrant green and lavender Candy Melts candy, separately, according to package directions. Remelt reserved candy. On parchment-covered cake board, use patterns and melted candy in cut decorating bags to pipe seaweed, coral and kelp in various colors. Make two or three extra of each shape to allow for breakage. Chill until firm, about 3 to 5 minutes. Reserve remaining melted candy.

Mold seashells. Melt pink Candy Melts candy according to package directions. Remelt reserved yellow and orange candy if needed. Use melted candy in cut decorating bag to pipe two colors of candy into seashells mold. Use toothpick to marbleize. Mold three shells for cake top and five shells for base of cake. Chill until firm, about 10 to 15 minutes.

Make layered cake. Prepare batter following recipe directions using 2 in. high pan. Bake and cool cake.

Prepare buttercream icing following recipe directions. Tint light blue. Stack and fill 2-layer cake. Layer and fill for a 4 in. high cake. Place cake on foil-wrapped cake board. Use spatula to ice cake smooth.

Assemble cake. Position shells on cake top and along bottom edge. Insert fish toppers at staggered heights. Use icing to attach sea plants to side of cake.