The place where the hull meets the deck on a boat or ship is called the "gunwale" (pronounced GUNN-ell). It is the upper edge of the side of the vessel where the hull and deck come together. The gunwale adds rigidity to the hull and is essentially the top edge or lip of the hull sides at the deck level.
This term is common in boating anatomy and useful for identifying the boundary line between the hull—the body of the boat that provides buoyancy—and the deck, which is the surface above the hull that people walk on or use.
