 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?
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. Submarine: A submarine (or simply sub) is a watercraft capable of independent operation underwater. Submarine:  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).
ships