Slice the tops and bottoms off the cakes to smooth the surface and remove any excess browning.
Stack the cakes and spread some buttercream between each layer.
Coat the cake in a thin layer of buttercream, called a crumb coat. This will catch any excess cake crumbs. Chill the cake in the fridge for 20 minutes.
Divide the buttercream into 4 bowls. Use more blue and black food coloring to give the buttercream an ombre effect, leaving one bowl light blue and dye the remaining 3 to make them progressively darker. I added a couple drops of black food coloring, along with the blue food coloring, to the last 2 bowls, to give them a murky, dark sea water effect.
Place all the buttercream into individual piping bags fitted with a large, angled piping tip. I like the #127 piping tip.
Start with the darkest shade of buttercream. Hold the piping bag horizontally to the cake so that the wider end of the piping tip is against the cake and the narrow end is not touching the cake. Pipe the buttercream, starting at the base of the cake, in a horizontal line. As you pipe, wiggle the bag vertically, creating wavy patterns. Continue working your way up the cake, until it is ¼ covered. Switch to the next lighter shade, continue another ¼ of the cake, then use the next lighter shade and so on.
Once all sides of the cake have been covered, continue piping the lightest buttercream on the top of the cake by piping a spiral instead of stripes, starting from the center of the cake and working outwards.
Stick the tentacles onto the cake and enjoy!