What type of vessel is a submarine?

Answer 1

A submarine is any vessel that can submerge and raise itself using air and water, the key of course that the number of dives must equal the number of surfacings to be successful. Though the most common submarines are for military use, there are commercial, research, and even tourist submarines, with differing levels of capability.

True submarines have the ability to remain submerged for long periods of time, with crew limitations being the only liability (food), unless the vessel is an unmanned remote probe, but they aren't typically referred to as submarines since they do not have independent operational capability - they're tethered to a pilot vessel.

Submarines are considered a major threat to any nation that uses ocean trade routes for commerce. During the Falklands War, the Argentine Navy found out how vulnerable it was to submarine warfare when their Cruiser, the General Belgrano (an old WW2 era US Navy Light Cruiser sold to them) was sunk with the loss of over 300 crew by the British Royal Navy Submarine HMS Conqueror. She remains the only ship to date ever sunk by a nuclear powered submarine. The losses from the sinking of the Belgrano were half of total Argentine Kia's in the war. The incident forced the Argentine Navy to keep the rest of their Navy in port for the duration of the war, having no viable Anti-Submarine Warfare capability.

Submarines are primarily used as anti-ship and anti-submarine warfare weapons, though they are also used as part of the U.S. Nuclear Deterrent (Ballistic Missile Submarines); as Cruise Missile launch platforms that can destroy land targets hundreds of miles away; and for delivering Special Operations teams covertly (SEAL's).