I want to quickly share with you how I made/constructed Landon's 2nd Birthday cake. It was really easy, and it made a super cute centerpiece.
I made the cake with two 8" round cake pans and then iced it using two cans of store-bought vanilla icing. I used Wilton food coloring (a little goes a long way, and Wilton food coloring has more depth/brightness than other brands) to make the icing aqua-colored. To give the icing more stiffness so it would take on the look of waves, I added approximately a cup of powdered sugar before icing the cake. Then iced the cake and used my butterknife to "swirl" the icing around.
Sailboat: You can be creative here and do your own thing with materials you have on hand. I used a paper cup from the party and cut the entire bottom two-thirds off of it. Then I pressed/squeezed it in half to create a boat shape. To make the sails, I used heavy paper/cardstock, cut it, and used E6000 (super glue) to attach it to a chopstick. At the top, I used left-over felt (but you could use paper) to create a flag and a number "2". Once again, I used E6000 to attach it.
Lastly, I pressed the bottom of the boat down into the top of the cake and then pressed the chopstick down into the cake, as well. Ta-da!