Preheat the oven to 350 degrees F. Line the bottom of two 9-inch round cake pans with parchment paper and grease well. Set aside.

In a large bowl, combine 1 box of cake mix with 1 cup of water, 3 whole eggs, and 1/2 cup of vegetable oil. Use a hand mixer to blend ingredients for 2 minutes. Divide batter evenly into two bowls. Tint one bowl of batter with 5-6 drops of royal blue gel food coloring to make a deep blue color. Tint the other bowl with 2-4 drops of royal blue gel food coloring to make a lighter blue color.

Bake cake layers for 29-34 minutes until a toothpick inserted into the center comes out clean.

Allow cakes to cool for 10 minutes in the pan then remove to a sheet of waxed paper to cool completely.

Repeat steps 1-4 for the additional box of cake mix but tint one bowl of cake batter using 2-4 drops of sky blue food coloring. Leave the other bowl of batter white. You should have a total of 4 cake layers when done. Allow the cake to cool completely.

Level cake layers using a serrated knife.

Make a double batch of vanilla buttercream frosting according to recipe directions. Divide frosting into three separate bowls - half of the frosting in one bowl, and 1/4 of the frosting in each of the other two bowls.

Add several drops of sky blue food coloring to the largest bowl until you get to a light blue shade. This color will be used between the cake layers and for the crumb coat as well as the top waves on the cake.

Take one of the smaller bowls of frosting and add 2-4 drops of royal blue food coloring until you get a medium shade of blue.

Take the final bowl of frosting and add 5-8 drops of royal blue food coloring until you get a deeper shade of blue.

Stack the darkest cake layer on the bottom and add the thin layer of light blue frosting. Stack the remaining cake layers from darkest to lightest, ending with the white cake layer. Add the light blue frosting in between each layer.

Add a crumb coat (thin layer of frosting) all over the top and sides of the cake with the light blue frosting. Place the cake into the refrigerator for at least 30 minutes to chill.

Add the darkest blue frosting to a piping bag that has been fitted with an open star tip, such as the Wilton 1M tip. Secure with a clip. Starting at the bottom of the cake, pipe waves all the way around. Pipe a second row of dark blue waves above the first row. Start the waves in between the waves so that they are staggered on each row.

Add the medium blue frosting to a piping bag that has been fitted with an open star tip, such as the Wilton 1M tip. Pipe two rows of medium blue waves on top of the dark blue ones, continuing to stagger them above the waves on the row below.

Add the remaining light blue frosting to a piping bag that has been fitted with an open star tip, such as the Wilton 1M tip. Pipe two or more rows of light blue waves on top of the medium blue ones, continuing to stagger them above the waves on the row below. Continue the light blue waves all the way to the top of the cake. It's okay if the waves extend past the top of the cake.

Use an offset spatula to add a layer of the light blue frosting to the top of the cake. If you have light blue frosting left over, you can pipe stars all the way around the top edge of the cake to fill any gaps from the waves.

Spoon brown sugar on half of the top of the cake to represent sand at the beach, The edges of the sand can be irregular like the shoreline of a beach. Press the brown sugar down with the back of the spoon.

Decorate with beach-themed decorations, such as palm trees, beach chairs, beach umbrellas, starfish, etc.