Prepare the cake mix according to the package directions, divide it between two 9-inch round pans, and bake at 350°F for 25 to 30 minutes. Cool completely on wire racks.
Trim the cake tops level, stack with 1 cup frosting between the layers, and chill for 15 minutes.
Apply a 1/2-cup crumb coat and chill for 10 minutes, then smooth 1 cup frosting over the cake.
Mix crushed graham crackers with brown sanding sugar; press around base and sprinkle on top for "sand."
Tint the remaining 1/2 cup frosting blue; pipe waves along the top edge and add blue dollops for water.
Use paper straws cut to varying heights for turrets; frost around them and pack with graham "sand."
Arrange gummy fish, shells, and shovel. Chill 30 minutes before serving.