Marine salvage in the context of "Standard diving dress"

A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or informally to refer to remotely operated vehicles and robots, or to medium-sized or smaller vessels (such as the midget submarine and the wet sub). Submarines are referred to as boats rather than ships regardless of their size.

Although experimental submarines had been built earlier, submarine design took off during the 19th century, and submarines were adopted by several navies. They were first used widely during World War I (1914–1918), and are now used in many navies, large and small. Their military uses include: attacking enemy surface ships (merchant and military) or other submarines; aircraft carrier protection; blockade running; nuclear deterrence; stealth operations in denied areas when gathering intelligence and doing reconnaissance; denying or influencing enemy movements; conventional land attacks (for example, launching a cruise missile); and covert insertion of frogmen or special forces. Their civilian uses include: marine science; salvage; exploration; and facility inspection and maintenance. Submarines can be modified for specialized functions such as search-and-rescue missions and undersea cable repair. They are also used in the tourism industry and in undersea archaeology. Modern deep-diving submarines derive from the bathyscaphe, which evolved from the diving bell.

A float (also called a pontoon) is an airtight hollow structure, similar to a pressure vessel, designed to provide buoyancy in water. Its principal applications are in watercraft hulls, aircraft floats, floating piers, aquaculture, pontoon bridges, and marine engineering applications such as salvage.

Underwater divers may be employed in any branch of an armed force, including the navy, army, marines, air force and coast guard.Scope of operations includes: search and recovery, search and rescue, hydrographic survey, explosive ordnance disposal, demolition, underwater engineering, salvage, ships husbandry, reconnaissance, infiltration, sabotage, counterifiltration, underwater combat and security.

Professional diving is underwater diving where the divers are paid for their work. Occupational diving has a similar meaning and applications. The procedures are often regulated by legislation and codes of practice as it is an inherently hazardous occupation and the diver works as a member of a team. Due to the dangerous nature of some professional diving operations, specialized equipment such as an on-site hyperbaric chamber and diver-to-surface communication system is often required by law, and the mode of diving for some applications may be regulated.

There are several branches of professional diving, the best known of which is probably commercial diving and its specialised applications, commercial offshore diving, inshore civil engineering diving, marine salvage diving, hazmat diving, and ships husbandry diving. There are also applications in scientific research, marine archaeology, fishing and aquaculture, public safety, law enforcement, military service, media work and diver training.

On 21 June 1919, shortly after the end of the First World War, the Imperial German Navy's High Seas Fleet was scuttled by its sailors while held off the harbour of the British Royal Navy base at Scapa Flow, in the Orkney Islands of Scotland. The fleet was interned there under the terms of the Armistice of 11 November 1918 while negotiations took place over its fate. Fearing that either the British would seize the ships unilaterally or the German government at the time might reject the Treaty of Versailles and resume the war effort (in which case the ships could be used against Germany), Admiral Ludwig von Reuter decided to scuttle the fleet.

Intervening British guard ships were able to beach some of the ships, but 52 of the 74 interned vessels sank. Many of the wrecks were salvaged over the next two decades and were towed away for scrapping. Those that remain are popular diving sites and a source of low-background steel.