Using a serrated knife, trim the tops of both cake layers to be level. Brush away excess crumbs. Place the bottom layer on an 8-inch cardboard square. Add food coloring, one drop at a time, to the frosting, and mix until desired color. Spread 1 1/2 to 2 cups tinted frosting over the top of the bottom layer. Top with the second layer, cut side down. Spread 2 cups frosting in a thin layer over the entire cake.
Place the assembled cake on an inverted cake pan to raise the bottom edge for easier decorating. Use a large offset spatula to generously cover all of the cake with the remaining frosting. Use the long edge of the spatula to smooth the frosting on all sides.
Press a cake-decorating comb into the sides to make vertical lines around the cake, dividing each side into four equal parts. Pull the comb or a serrated knife across the sides, making a U shape from one line to the next to form waves. On top of the cake, use a small offset spatula to pull the still-moist frosting straight up, creating frothy waves. (A paper sailboatĀ makes the perfect topper).
Decorate fish-shaped sugar cookies with royal icing, and press into the frosting on the sides of the cake. Serve slices of the cake with more fish cookies on the side.