five 6-inch round cake pans; 6 piping bags fitted with #104 decorator piping tips; a 3.5-inch round pastry ring or cookie cutter; a 2-inch round cookie cutter; a 9-inch-long cake dowel (about 1/8-inch diameter); kitchen gloves, optional
For the cake layers: Preheat the oven to 350 degrees F. Coat five 6-inch round cake pans with flour-based baking spray.
In the bowl of an electric mixer fitted with the whisk attachment, combine the flour, granulated sugar, cake mixes and salt. Whisk on low speed until combined. Add the sour cream, eggs and 2 cups water. Mix on medium speed until just combined. Stop the mixer and scrape down the sides of the bowl; add the vanilla extract. Mix again until no streaks of the dry ingredients remain.
Divide 1 cup of batter between two small bowls (1/2 cup in each); tint one bowl of batter with the blue food color and tint the other with the teal food color. Stir to combine. Add the food color a little at a time until a bright hue is achieved.
Divide the remaining white cake batter among the prepared pans. Spoon a little blue and teal batter into each pan and swirl the colors together with a skewer or butter knife. Bake the cakes until they are lightly browned and a toothpick tester inserted near the center comes out clean, about 30 minutes. Let the cakes cool completely. Level each cake layer using a serrated knife or a wire cake leveler.
For the Swiss meringue buttercream: Set a saucepan filled one-third full of water over medium-high heat and bring to a simmer.
Whisk together the egg whites, granulated sugar and salt in a large heatproof bowl. Set over the simmering water and whisk until the mixture is hot to the touch and the sugar has dissolved.
Transfer the mixture to the bowl of a stand mixer filled with the whisk attachment. Beat on low speed for 2 minutes. Increase to medium high and beat until stiff peaks are formed. Continue beating at medium-high speed until the mixture is fluffy and has cooled, about 5 minutes (the mixing bowl should feel cool to the touch).
Turn the mixer off and switch from the whisk to the paddle attachment. Turn the mixer on medium low and add the butter a few cubes at a time, beating until well incorporated before the next addition. Add the vanilla extract. Beat until the frosting is thick and completely smooth.
Remove about 1/2 cup of the frosting to each of 6 bowls. Tint one bowl navy blue using the navy blue food color, tint another bowl dark blue with the blue food color, tint another bowl dark teal using the teal food color, tint another bowl light teal using the teal food color and tint the fifth bowl light blue using the blue food color. The last bowl should remain untinted. Transfer each of the colors to separate piping bags, each fitted with a #104 decorator piping tip.
For the marshmallow fondant shark fins: Place the marshmallows and 1 tablespoon water in a microwave-safe bowl. Microwave for 1 minute at 100% power, or until the marshmallows puff out of shape. Stir the mixture until the marshmallows deflate.
Add the powdered sugar 1 cup at a time, kneading the mixture well with your hands (kitchen gloves make this job less messy). Keep adding the powdered sugar until the mixture is thick, smooth and pliable. You may not have to use all of the powdered sugar.
Add a drop of black food color and a drop of blue food color to the fondant and knead until a dark blue-grey color is achieved.
Roll the fondant flat on a work surface lightly dusted with powdered sugar. Stamp two 3.5-inch circles from the fondant using a pastry ring or cookie cutter. Cut each circle into quarters. Use a 2-inch round cookie cutter to trim away 1/8 inch from a flat side of each piece, creating a shark fin shape. Transfer the shark fins to a baking sheet and let dry until firm and rigid and the fins can stand upright, 1 to 2 hours.
For the frosting and assembling: Use the remaining untinted frosting to fill the cake layers. Apply a thin even layer of buttercream to the cake's exterior and insert a 9-inch dowel through the top center of the cake to anchor the layers together. Chill until firm, about 20 minutes.
Use the back of a butter knife to score lines around the cake to divide it into 6 sections (2 1/2- to 3-inch sections). Begin piping frosting at the bottom of the cake using the navy blue buttercream. Hold the piping bag so that the smaller end of the decorator tip is positioned away from the cake. Pipe lines of buttercream close together, traveling towards the top of the cake to the first score line. Repeat the process using the remaining colors in this order: dark blue, dark teal, light teal, light blue, white (untinted).
Using the leftover navy buttercream in the piping bag, pipe enough frosting on top of the cake to cover it; use an offset spatula to create swirls resembling choppy ocean water. Place the set fondant shark fins around the top edge of the cake in a circle. Sprinkle on white nonpareil "bubbles" and place white sugar pearls at random on top of the cake and around the fins. Serve tall slices of cake at room temperature on large plates.