What defines the outer shell of the ship that holds water out?

The hull is fundamental to a ship's structure, as it forms the outer shell that provides the primary capability to keep water out. It encompasses the entire body of the vessel, extending from the keel at the bottom to the deck at the top. The hull's shape and design are crucial for providing buoyancy, stability, and overall seaworthiness, effectively allowing the ship to navigate through water without taking on excess water itself.

In this context, other terms have different meanings. For example, the waterline refers to the horizontal line that marks the surface of the water against the side of the ship. Freeboard is the vertical distance from the waterline to the upper deck level; it essentially indicates how much of the hull remains above the water and is a measure of safety against water ingress. Beam refers to the width of the ship at its widest point. While each of these terms is essential in maritime terminology, they do not define the outer shell that is specifically responsible for holding water out as the hull does.