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 using the arrival by water to assault land targets, drilling 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, anti-drug, law enforcement, anti-terrorism, and anti-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. The article "Riding on Proton" by Afonchenko (in Russian) may describe in passing a Soviet Bloc frogman infiltration into South Korean sea.
Clandestine underwater work, e.g.:
Recovering underwater objects.
Clandestine fitting of monitoring devices on underwater communication cables in enemy waters.
Investigating unidentified divers, or a sonar echo that may be unidentified divers. Diving sea-police work may be included here. See anti-frogman techniques.
Checking ships, boats, structures, and harbors for limpet mines and other sabotage; and ordinary routine maintenance in war conditions. If the inspection divers during this find attacking frogmen laying mines, this category may merge into the previous category.
Underwater mine clearance and bomb disposal.
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 the perpetrator of which 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. Installing eavesdropping devices is the best example. 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: e.g. ("...the secretary will disavow any knowledge of your actions.")