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?
Sentence (from "Submarine"): 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.
Relevant evidence: No