Question:  The Ordnance QF Hotchkiss 6 pounder gun were introduced to defend against watercraft capable of independent operation underwater usually referred to as "boats" rather than what?

Context paragraphs:
QF 2-pounder naval gun: The 2-pounder gun, officially designated the QF 2-pounder (QF denoting "quick firing") and universally known as the pom-pom, was a 40 mm British autocannon, used as an anti-aircraft gun by the Royal Navy.  The name came from the sound that the original models make when firing.  This QF 2-pounder was not the same gun as the Ordnance QF 2 pounder, used by the British Army as an anti-tank gun and a tank gun; they shared only the projectile weight of 2 pounds.

Ordnance BLC 15-pounder: The Ordnance BLC 15 pounder gun (BLC stood for BL Converted) was a modernised version of the obsolete BL 15 pounder 7 cwt gun, incorporating a recoil and recuperator mechanism above the barrel and a modified quicker-opening breech.  It was developed to provide Territorial Force artillery brigades with a reasonably modern field gun without incurring the expense of equipping them with the newer 18 pounder.  It is the gun that writers usually mean by "15 pounder gun" in World War I, but can be confused with the earlier Ordnance QF 15 pounder "Ehrhardt" or Ordnance BL 15 pounder, both of which fired the same shell.

Ordnance QF 75 mm: The Ordnance QF 75 mm, abbreviated to OQF 75 mm, was a British tank-gun of the Second World War.  It was obtained by boring out the Ordnance QF 6 pounder ("6 pdr") 57-mm anti-tank gun to 75-mm, to give better performance against infantry targets in a similar fashion to the 75mm M3 gun fitted to the American Sherman tank.  The QF came from "quick-firing", referring to the use of ammunition with the shell and propellant in a single cartridge.  The gun was also sometimes known as ROQF from Royal Ordnance (the manufacturer) Quick-Firing.

Submarine: A submarine (or simply sub) is a watercraft capable of independent operation underwater.  It differs from a submersible, which has more limited underwater capability.  The term most commonly refers to a large, crewed vessel.  It is also sometimes used historically or colloquially to refer to remotely operated vehicles and robots, as well as medium-sized or smaller vessels, such as the midget submarine and the wet sub.  The noun "submarine" evolved as a shortened form of "submarine boat"; by naval tradition, submarines are usually referred to as "boats" rather than as "ships", regardless of their size ("boat" is usually reserved for seagoing vessels of relatively small size).

QF 6-pounder Hotchkiss: The Ordnance QF Hotchkiss 6 pounder gun Mk I and Mk II or QF 6 pounder 8 cwt were a family of long-lived light 57 mm naval guns introduced in 1885 to defend against new, small and fast vessels such as torpedo boats and later submarines.  There were many variants produced, often under license which ranged in length from 40 to 58 calibers, but 40 caliber was the most common version.

Ordnance QF 25-pounder Short: The Ordnance QF 25-pounder Short was an Australian variant of the British Ordnance QF 25-pounder field gun/howitzer.  The gun was developed by modifying the 25-pounder's design to improve its mobility during jungle warfare.  Development began in 1942, and the weapon first entered service with the Australian Army the next year.  It was used by several Royal Australian Artillery regiments during fighting in the South West Pacific Area, before being declared obsolete in 1946.

QF 6-pounder 6 cwt Hotchkiss: The Ordnance QF 6-pounder 6 cwt Hotchkiss Mk I and Mk II was a shortened version of the original QF 6 pounder Hotchkiss naval gun, and was developed specifically for use in the sponsons of the later Marks of British tanks in World War I, from Mark IV onwards.

Ordnance QF 15-pounder: The Ordnance QF 15 pounder gun, commonly referred to as the "Ehrhardt", was a modern German field gun purchased by Britain in 1900 as a stopgap measure to upgrade its field artillery to modern QF standards, while it developed its own alternative.  This was precipitated by the experience of the British Army in South Africa during the Boer War, where its standard field gun, the BL 15 pounder, was out-performed by modern French and German field guns deployed by the Boers.  It bore no relation to the BL 15 pounder or BLC 15 pounder, two other guns in British service at the time, other than a common shell.

OQF 3-pounder gun: The Ordnance QF 3 pounder gun was a 47 mm British tank gun based on the Ordnance QF 3 pounder Vickers naval gun, mounted on Vickers Medium Tanks in the 1920s and 1930s.  The gun was produced in 32 calibre (59 inch) and 40 calibre (74 inch) versions.

Ordnance QF 2-pounder: The Ordnance QF 2-pounder (QF denoting "quick firing"), or simply "2 pounder gun", was a 40 mm British anti-tank and vehicle-mounted gun, employed in the Second World War.  It was actively used in the Battle of France, and during the North Africa Campaign.  As Axis tanks improved in armoured protection, it lost effectiveness, and it was gradually replaced by the 6-pounder, starting in 1942, though some remained in service until the end of the war.  In its vehicle-mounted variant, the 2-pounder was also a common main gun on British tanks early in World War II, and was a typical main armament of armoured cars, such as the Daimler, throughout the war.

Supporting paragraphs: QF 6-pounder Hotchkiss Submarine
Answer: ships