OVERVIEW: A combat swimmer is a military operator trained for underwater and surface infiltration, reconnaissance, sabotage, and direct action. Also known as frogmen, they employ scuba or closed-circuit rebreathers and tactical swimming techniques to conduct covert maritime operations.

DETAILS: Combat swimmer units exist in various forces including U.S. Navy SEALs, German Kommando Spezialkräfte Marine (KSM), French Commando Hubert, and Danish Frogman Corps as elite maritime special forces specializing in amphibious warfare, underwater demolitions, and maritime counter‑terrorism. Training includes long-distance fin swims, underwater navigation with compass boards, dive chamber and rebreather familiarization, obstacle clearance, and mission-specific combat techniques. They use weapons, explosives, and deployment methods such as helo-cast, submarine lockout, and specialized craft like Mark V SOCs for insertion/extraction. Combat swimmers operate in units of 4-8 divers with roles including medic, communications, and demolitions, capable of executing clandestine operations in hostile maritime environments.

FUNCTION: Using closed‑circuit rebreathers, combat swimmers move silently underwater to approach targets undetected, conduct reconnaissance, place limpet mines, sabotage vessels or port facilities, block channels, and support hostage rescue missions. They can also facilitate amphibious assaults, beach reconnaissance for landings, and maritime interdiction, often coordinating with naval and air support.

NOTE:
IOC: Varies by unit (e.g., German KSM 1958; Danish Frogman Corps 1957; U.S. SEALs lineage from WWII UDT)
Operators: United States (Navy SEALs, Marine Raiders, Army Special Forces, Air Force PJs/PJs), Germany (KSM), France (Commando Hubert), Denmark (Frogman Corps), others
Platforms: Submarines (lockout), small craft (Mark V, combatant boats), helicopters (helo-cast), surface ships
Conflict used in: engagements include D-Day UDT missions (1944), Gulf War boarding ops, global counter-piracy, maritime counter-terrorism, and modern high-value raids

SOURCE: Military.com on Combat Side Stroke & diver training; U.S. Army on Combat Diver Qualification Course; Business Insider on U.S. special-ops diving; Wikipedia: Kommando Spezialkräfte Marine; Wikipedia: Frogman Corps Denmark; Wikipedia: Combat sidestroke; Wikipedia: Mark V Special Operations Craft