The term 'bottom of the sea' refers to the lowest part of a body of water, usually the ocean. In nautical terms, the KEEL is the structural backbone of a ship, running from the bow to the stern and providing stability. The HULL is the outer shell of a vessel, which encloses the interior and protects it from the elements. Together, the KEEL and HULL form the bottom of the sea for a ship, providing both structure and protection.