A sailor who is far out at sea has to hope that the hull of her boat is watertight. The hull of a boat or a ship is its outer body.
A boat's hull can be made of wood, metal, or fiberglass — it gives a boat or ship its shape and keeps water out. Another kind of hull is the outer covering of a seed. Corn, for example, has a hull that is removed when you husk it and reveal the kernels. This kind of hull is thought to be the root of the nautical hull, probably based on the similarity between a ship's hull and an open pea pod.