This is a bit of old news, but I entered my first attempt at a sandcastle cake in the June 2011 Marin County Fair and took 2nd place! To be honest, there were not a huge number of cakes entered, but I'm still proud of my award-winning Nabisco creation.
In order to create a sturdy canvas, I covered the cakes with fondant. Then I spread buttercream on top and pressed a mixture of pulverized brown sugar, graham crackers, and Nilla wafers into the frosting. The little sea creature buddies and flag were molded from fondant, and I used royal icing to make the seaweed. All in all, it was a fun cake to make!