Marines
History: In the early days of naval warfare, there was little distinction between sailors and soldiers on a warship. The oarsmen of Ancient Greek and Ancient Roman ships had to be capable of fighting the rowers of opposing ships hand-to-hand; though hoplites began appearing on Greek ships specifically for the boarding of enemy ships.
The Roman Republic was the first to understand the importance of professional soldiers dedicated to melee combat onboard of ships. During the First Punic War, Roman crews remained inferior in naval experience to the Carthaginians and could not hope to match them in naval tactics, which required great fleet maneuverability and tactical experience. The Romans therefore employed a novel weapon which changed sea warfare to their advantage — they equipped their ships with the corvus, a long pivoting plank with a beak-like spike on the underside for hooking onto enemy ships, possibly developed earlier by the Syracusans against the Athenians during the Sicilian Expedition of the Peloponnesian War. Using it as a boarding bridge, Roman infantrymen were able to invade an enemy ship, transforming sea combat into a version of land combat, where the Roman legionaries had the upper hand. During the early Principate, a ship's crew, regardless of its size, was organized as a centuria. Crewmen could sign on as naval infantry (called Marinus), rowers/seamen, craftsmen and various other jobs, though all personnel serving in the imperial fleet were classed as milites ("soldiers"), regardless of their function; only when differentiation with the army was required, were the adjectives classiarius or classicus added. The Roman Navy's two fleet legions, I Adiutrix and II Adiutrix, were among the first distinct naval infantry units.
The first organized marine corps was created in Venice by the Doge Enrico Dandolo when he created the first regiment of ten companies spread on several ships. That corps participated in the conquest of Byzantium (1203–1204), later officially called "Fanti da Mar" (sea infantry) in 1550., Venice also had dedicated naval expeditionary corps of naval infantry recruited primarily from Dalmatia from the local population called the Oltremarini (overseas troops) 
Later, Spanish King Carlos I assigned the naval infantry of the Compañías Viejas del Mar de Nápoles (Naples Sea Old Companies) to the Escuadras de Galeras del Mediterráneo (Mediterranean Galley Squadrons) in 1537, progenitors of the current Spanish Navy Marines (Infantería de Marina) corps, making them the oldest marine corps still in active service in the world.

Etymology and translations: The English noun marine is from the adjective marine, meaning "of the sea", via French marin ("of the sea") from Latin marinus ("of the sea") itself from mare ("sea"), from Proto-Indo-European *móri ("body of water, lake") (cognate with Old English mere ("sea, lake"), Dutch meer, German Meer, all from Proto-Germanic *mari).
The word marine was originally used for the marine-type forces of England; however, the word marine or marina means "navy" in many European languages, including Dutch, French, Italian, German, Spanish, Danish, and Norwegian.
Because of this, exact one-word translations for the English term "marines" do not exist in many other languages (with the notable exception of the Dutch word marinier). This can lead to misunderstandings when translating. Marine forces in non-English speaking countries typically have names that translate in English to naval infantry or coastal infantry. In French-speaking countries, two phrases exist which could be translated as marine, "troupes de marine" and "fusiliers-marins"; similar phrases exist elsewhere, e.g., in Portuguese Fuzileiros Navais.

Roles: The principal role of marine troops is military operations in the littoral zone; operating from ships they are trained to land on and secure key points to around 85 km (or 50 miles) inland, or as far as ship borne logistics can provide.
Marine units primarily deploy from warships using boats, landing craft, hovercraft, amphibious vehicles or helicopters. Specialist units are also trained in combat diving/combat swimming and parachuting.
As well as amphibious operations, marine troops are used in a variety of other, naval roles. Stationed at naval bases or forming marine detachments on board naval ships, they also conduct small scale raiding, maritime boarding operations, security of naval vessels and bases, riverine and coastal missions, mess duty, and field day operations.
In addition to their primary roles, they perform other tasks, including special operations and land warfare, separate from naval operations; ceremonial duties and other miscellaneous tasks as directed by their governments.

By country: 

Algeria: The Marine Fusilier Regiments are the marine infantry regiments of the Algerian Navy and they are specialised in amphibious warfare.
The RFM have about 7000 soldiers in their ranks. Established in 1985.

Argentina: The Argentine Marine Corps (Infantería de Marina de la Armada de la República Argentina or IMARA) is a part of the Argentine Navy. Argentine marines have the same rank insignia and titles as the rest of the navy, although enlisted personnel have their own parade uniform. The Argentine Marine Corps dates from 1827 when a single infantry battalion was raised. This was expanded in 1880, but seven years later, the corps was merged with the existing coast artillery, to form a Naval Artillery Regiment. A series of reorganizations followed until responsibility for coastal defense was passed to the Argentine Army in 1898. Between 1935 and 1938 the marines reappeared in the form of five battalions of Marine Infantry, serving both on board ship and in coastal defense fortifications. In 1968, the Infantería de Marina was reorganized as a separate corps within the Navy.

Australia: The marine and naval infantry designations are not applied to Australian Defence Force units, although some Australian Army units specialise in amphibious warfare, including 2nd Battalion, Royal Australian Regiment which has provided an amphibious light infantry role from 2012.

Bahamas: The Royal Bahamas Defence Force (RBDF) is the navy of The Bahamas. Since the Bahamas does not have an army or an air force, its navy composes the entirety of its armed forces. The RBDF Commando Squadron is a sizable force of 500 Special Marine Commandos.

Bangladesh: The Special warfare Driving and Salvage (SWADS) is special operations force of the Bangladesh Navy. SWADS is trained for the role of naval infantry and it consists of elite soldiers specially chosen form the national armed forces branches. They receive special training in the United States.

Bolivia: Even though Bolivia is landlocked, Bolivian politics have always aspired to regain its coastline from Chile, after losing access to the Pacific coast in the 1879-1880 war with Chile. Because of that, Bolivia still maintains a naval force. The Bolivian Naval Force includes about 2,000 naval infantry personnel and marines. These are organized into seven small battalions.

Brazil: The Corps of Naval Fusiliers (Corpo de Fuzileiros Navais) is subordinate to the Brazilian Navy. The marine corps is composed of an operational brigade and some guard and ceremonial duty battalions. The main unit is the brigade-sized Divisão Anfíbia (Amphibious Division). Officers´ ranks and titles are the same as for the rest of the Navy, although officers wear a star above the stripes, instead of the loop worn by surface officers.

Cambodia: During the 1970-75 Cambodian Civil War the Cambodian Marine Corps were active but were effectively disbanded by the end of the Cambodian–Vietnamese War. The Royal Cambodian Navy created a force of 2,000 marines in 2007 known as the 31st Naval Infantry Brigade

Canada: Canada had a history of participating in amphibious operations such as the Normandy landings and the Allied invasion of Sicily. Even though Canada does not have a marine corps, it has units that can carry out Marines-type operations, such as an amphibious-operations trained company of the 3rd Battalion of the Royal 22nd Regiment, JTF2 that specializes in combat diving and amphibious reconnaissance, the Naval Tactical Operations Group that specializes in maritime interdiction, and the Naval Security Team that can provide force protection for amphibious forces.
Several authors have written a number of articles in various Canadian professional military journals since 2019 proposing/discussing the creation of a Canadian Arctic amphibious capability, including the adaption of one to three Canadian Army infantry battalions to provide the required landing forces.

Chile: The Chilean Marine Corps is a branch of the Chilean Navy. Specialized in amphibious assaults, the corps is built around four detachments based along Chile's long coasts at Viña del Mar, Talcahuano, Punta Arenas, and Iquique. There are also a number of independent companies and platoons, for security protection at naval bases, other shore installations and the Ministry of Defense. The Viña del Mar and Talcahuano detachments contribute to the Amphibious Expeditionary Brigade (Brigada Anfibia Expedicionaria).