Frogman
Scope of operations: Tactical diving is a branch of professional diving carried out by armed forces and tactical units. They may be divided into:

Combat or assault divers.
Special mission work divers (called "clearance divers" in the British Royal Navy and Royal Australian Navy), who do general work underwater.
Work divers who are trained in defusing mines and removing other explosives underwater.
These groups may overlap, and the same men may serve as assault divers and work divers, such as the Australian Clearance Diving Branch (RAN).
The range of operations performed by these operatives includes:

Amphibious assault: stealthy deployment of land or boarding forces. The vast majority of combat swimmer missions are simply to get "from here to there" and arrive suitably equipped and in sufficient physical condition to fight on arrival. The deployment of tactical forces by water to assault land targets, oil platforms, or surface ship targets (as in boardings for seizure of evidence) is a major driver behind the equipping and training of combat swimmers. The purposes are many, but include feint and deception, counter-drug, law enforcement, counter-terrorism, and counter-proliferation missions.
Sabotage: This includes putting limpet mines on ships.
Clandestine surveying: Surveying a beach before a troop landing, or other forms of unauthorized underwater surveying in denied waters.
Clandestine underwater work, e.g.:
Recovering underwater objects.
Clandestine fitting of monitoring devices on submarine communications cables in enemy waters.
Investigating unidentified divers, or a sonar echo that may be unidentified divers. Police diving work may be included.
Checking ships, boats, structures, and harbors for limpet mines and other sabotage; and ordinary routine maintenance in war conditions.
Underwater mine clearance and bomb disposal.
Typically, a diver with closed circuit oxygen rebreathing equipment will stay within a depth limit of 20 feet (6.1 m) with limited deeper excursions to a maximum of 50 feet (15 m) because of the risk of seizure due to acute oxygen toxicity. The use of nitrox or mixed gas rebreathers can extend this depth range considerably, but this may be beyond the scope of operations, depending on the unit.

Mission descriptions: US and UK forces use these official definitions for mission descriptors:

Stealthy
keeping out of sight (e.g., underwater) when approaching the target.
Covert
carrying out an action of which the enemy may become aware, but whose perpetrator cannot easily be discovered or apprehended. Covert action often involves military force which cannot be hidden once it has happened. Stealth on approach, and frequently on departure, may be used.
Clandestine
it is intended that the enemy does not find out then or afterwards that the action has happened – for example, installing eavesdropping devices. Approach, installing the devices, and departure are all to be kept from the knowledge of the enemy. If the operation or its purpose is exposed, then the actor will usually make sure that the action at least remains "covert", or unattributable.

Defending against frogmen: Anti-frogman techniques are security methods developed to protect watercraft, ports and installations, and other sensitive resources both in or nearby vulnerable waterways from potential threats or intrusions by frogmen.

Equipment: Frogmen on clandestine operations use rebreathers, as the bubbles released by open-circuit scuba would reveal them to surface lookouts and make a noise which hydrophones could easily detect.

Origins of the name: A few different explanations have been given for the origin of the term frogman.

Paul Boyton adopted the stage name The Fearless Frogman. In the 1870s, he was a long distance swimmer who wore a rubber immersion suit, with hood.
In an interview with historian Erick Simmel, John Spence claimed that the name "frogman" was coined while he was training in a green waterproof suit, "Someone saw me surfacing one day and yelled out, 'Hey, frogman!' The name stuck for all of us."

History: In ancient Roman and Greek times, there were instances of men swimming or diving for combat, sometimes using a hollow plant stem or a long bone as a snorkel. Diving with snorkel is mentioned by Aristotle (4th century BC). The earliest descriptions of frogmen in war are found in Thucydides' History of the Peloponnesian War. The first instance was in 425 BC, when the Athenian fleet besieged the Spartans on the small island of Sphacteria. The Spartans managed to get supplies from the mainland by underwater swimmers towing submerged sacks of supplies. In another incident of the same war, in 415 BC, the Athenians used combat divers in the port of Syracuse, Sicily. The Syracuseans had planted vertical wooden poles in the bottom around their port, to prevent the Athenian triremes from entering. The poles were submerged, not visible above the sea level. The Athenians used various means to cut these obstacles, including divers with saws. It is believed that the underwater sawing required snorkels for breathing and diving weights to keep the divers stable.
Also, in the writings of Al-Maqrizi, it is also claimed that the naval forces of the Fatimid Caliphate, in an engagement with Byzantine forces off the coast of Messina henceforth referred to as the Battle of the Straits, employed a novel strategy with strong similarities to modern-day frogmen tactics. In the writings of Heinz Halm, who studied and translated the writings of Al-Maqrizi and other contemporary Islamic historians, it is described: "They would dive from their own ship and swim over to the enemy ship; they would fasten ropes to its rudder, along which earthenware pots containing Greek fire were then made to slide over to the enemy ship, and shattered on the sternpost." Apparently, this tactic succeeded in destroying many Byzantine vessels, and the battle ended in a major Fatimid victory; according to the Arab historians, a thousand prisoners were taken, including the Byzantine admiral, Niketas, with many of his officers, as well as a heavy Indian sword which bore an inscription indicating that it had once belonged to Muhammad.
The Hungarian Chronicon Pictum claims that Henry III's 1052 invasion of Hungary was defeated by a skillful diver who sabotaged Henry's supply fleet. The unexpected sinking of the ships is confirmed by German chronicles.
On 4 November 1918, during World War I, Italian frogmen sunk the Austro-Hungarian ship Viribus Unitis.
Italy started World War II with a commando frogman force already trained. Britain, Germany, the United States, and the Soviet Union started commando frogman forces during World War II.

First frogmen: The word frogman appeared first in the stage name The Fearless Frogman of Paul Boyton, who since the 1870s broke records in long distance swimming to demonstrate a newly invented rubber immersion suit, with an inflated hood.
The first modern frogmen were the World War II Italian commando frogmen of Decima Flottiglia MAS (now "ComSubIn": Comando Raggruppamento Subacquei e Incursori Teseo Tesei) which formed in 1938 and was first in action in 1940. Originally these divers were called "Uomini Gamma" because they were members of the top secret special unit called "Gruppo Gamma", which originated from the kind of Pirelli rubber skin-suit nicknamed muta gamma used by these divers. Later they were nicknamed "Uomini Rana," Italian for "frog men", because of an underwater swimming frog kick style, similar to that of frogs, or because their fins looked like frog's feet.
This special corps used an early oxygen rebreather scuba set, the Auto Respiratore ad Ossigeno (A.R.O), a development of the Dräger oxygen self-contained breathing apparatus designed for the mining industry and of the Davis Submerged Escape Apparatus made by Siebe, Gorman & Co and by Bergomi, designed for escaping from sunken submarines. This was used from about 1920 for spearfishing by Italian sport divers, modified and adapted by the Italian navy engineers for safe underwater use and built by Pirelli and SALVAS from about 1933, and so became a precursor of the modern diving rebreather.
For this new way of underwater diving, the Italian frogmen trained in La Spezia, Liguria, using the newly available Genoese free diving spearfishing equipment; diving mask, snorkel, swimfins, and rubber dry suit, the first specially made diving watch (the luminescent Panerai), and the new A.R.O. scuba unit. This was a revolutionary alternative way to dive, and the start of the transition from the usual heavy underwater diving equipment of the hard hat divers which had been in general use since the 18th century, to self-contained divers, free of being tethered by an air line and rope connection.

Wartime operations: After Italy declared war, the Decima Flottiglia MAS (Xª MAS) attempted several frogmen attacks on British naval bases in the Mediterranean between June 1940 and July 1941, but none were successful, because of equipment failure or early detection by British forces. On September 10, 1941, eight Xª MAS frogmen were inserted by submarine close to the British harbour at Gibraltar, where using human torpedoes to penetrate the defences, sank three merchant ships before escaping through neutral Spain.