I have a 4-year-old who is crazy about sharks, so I had to make him a shark birthday cake this year. I got some inspiration from this site and a few others, as well as looking for a real pic of a Great White through Google Images.
It is made with a regular chocolate cake mix from the store, as well as 2 pots of store-bought white icing. I used one pot for the blue icing, which I applied first (I used wax paper along the sides of the cake to keep the edges smudge-free). Then I used a toothpick to outline the text and the shark (whose outline I copied from a real pic of a shark).
Then I made my grey icing (one drop each of red, green, and blue into 1/3 of a pot of white icing), and piped it on to outline the shark - not including the side fins. Then I piped on the white underbelly, as well as the bubbles. I smoothed down all the fill in icing with a knife that I kept dipping in water, but shake off all the excess water each time! That way the icing doesn't stick to your knife and it gets nice and smooth.
After the white was on, I added the fins to give a bit of a 3D look. The bottom of the cake is rock candy, and the seaweed is piped-on green (made with mostly green food coloring, with a drop of red and yellow to tone it down a bit). I added the eye, gills, and inner mouth with black-ish icing that I mixed in a very small amount (1 tsp white icing, with 2-3 drops each of green and red coloring). I applied it with a toothpick.
When it was mostly dry, I added the teeth with tiny bits of white icing, again with a toothpick.
Well, it seems a shame that tomorrow we will cut the whole thing up! Hope this helps anyone else out there with a shark-lover in their house!