Recently we made a sandcastle beach cake for the graduating class. I've never worked with gel before so I was a little apprehensive as to how it would turn out. This is how we did it.
I started out with two layers of sheet cake. I put them together with frosting in the middle. I crumb coated it with white frosting. For the sand, I used the ninja and blended up graham cracker crumbs. I made the castle with a castle shaped pan with regular yellow cake. I sat the castle on the cake and then put the crumbs where I wanted them. I spread more frosting on the area with the water and made a sweeping effect with my spatula to make the wave effect. Then I laid the gel on top of the frosting I just spread. I left a little white frosting showing at the shore of the cake to make it look like sea foaming waves rolling up to shore. For the wet looking sand at the shore, I added some cinnamon and brown sugar to the graham cracker crumbs. I just kept mixing in until I liked the color. It tasted amazing by the way. My mother in law was a great help in making this cake as well as taking a few hours to mold and paint the fondant shells.
One issue we had was that the gel cracked and looked a lot different as we had no choice but to put it in the fridge as we had to make it early as it is to hot here and it could not sit out. Even though it cracked, it still looked like water. If we had used the gel for anything else, it would have turned out bad. Even with the little mishap, everyone loved it and devoured the entire thing!