Final answer:
The extreme width of a ship is correctly referred to as the beam, a key measurement in ship design affecting stability and maneuverability.
Step-by-step explanation:
The proper term for the extreme width of a ship is beam. This term is used to describe the widest point of the ship, which is generally in the middle of the vessel, from one side to the other. The beam is a critical measurement as it affects the ship's stability and ability to navigate through various waterways.
Other terms mentioned are related to different aspects of a ship's structure and function. The draft of a ship is the vertical distance between the waterline and the bottom of the hull (keel), the keel is the structural backbone of the ship running along the bottom, and the hull is the watertight body of the ship, which includes the beam but encompasses the entire lower portion of the ship.