Take off the shoebox lid and keep it aside. Paint the inside and outside of the shoebox with blue acrylic paint to represent the ocean. Allow the paint to dry completely.
Work the white modeling clay with your hands until it is malleable. Sculpt the clay into a mesa-like shape with steep sides and a broad, flat top -- akin to a thick piece of toast or a deck of cards. Use a toothpick to scratch lines into the sides of the structure. Measure the clay's thickness with a ruler. Stop sculpting when the thickness reaches 1 inch. Set the structure aside.
Knead the blue modeling clay until it is easy to work with. Model the the clay into an elongated, bulbous shape. Flatten one end, but manipulate the other end into a rounded form, much like a tall cactus or a cucumber. Pinch the clay into bumps and crags, using the toothpick to carve in fine lines and details. Measure the structure, ensuring it is 7 inches thick.
Stand the shoebox on end. Affix the white clay structure, which represents the iceberg above water, on top of the outside of the shoebox. Secure the above-water iceberg part to the box with superglue for added stability.
Adhere the blue clay structure, which reflects the iceberg below water, to the inside of the shoebox. Use superglue to fasten the flat part of the underwater iceberg to the underside of the top of the shoebox. Line up the blue structure to be exactly below the white structure. Let the glue dry completely.