What term describes the lowest part of the hull structure of a ship that extends below the waterline?

The term that describes the lowest part of the hull structure of a ship, which extends below the waterline, is the keel. The keel serves as the backbone of the ship and runs along the bottom of the hull from the bow (front) to the stern (back). It is a critical component of the vessel's structure, providing stability and strength. The shape and design of the keel influence the ship's ability to maneuver in the water and affects its overall seaworthiness.

The draft refers to the depth of water a ship requires to float safely, measured from the waterline to the lowest point of the hull, which is not the same as the keel itself. Freeboard is the vertical distance from the waterline to the upper deck line, indicating how high a vessel’s deck sits above the water, and is also unrelated to the part that extends below the waterline. The forecastle is the upper deck of a ship at the bow, which does not refer to any part of the hull structure extending below the waterline.