Delfin is a reported or supposed name for a Russian commando frogmen unit.
The Soviet Union started frogman operations during World War II. In 1941 the first unit of combat divers (or RON team) was formed in Leningrad.
SpN PDSS (Противо-Диверсионные Силы и Средства, Protivo-Diversionnye Sily i Sredstva; Anti-diversionary forces and means) are units of a special purpose anti-sabotage divers, which appeared in the Soviet Navy in the end of the 1960s to counter possible attacks of frogmen. Their name in full is (о) СпН ПДСС, [отряды] Специального назначения по борьбе с подводными диверсионными силами и средствами ((o) SpN PDSS, [otryady] Spetsial'nogo naznacheniya po bor'be s podvodnymi diversionnymi silami i sredstvami; Special-purpose combat [forces] with underwater diversionary forces and facilities).Before 2001/2000 they were called ОБ ПДСС, Отряды по борьбе с подводными диверсионными силами и средствами (OB PDSS, Otryady po bor'be s podvodnymi diversionnymi silami i sredstvami; Combat forces with underwater diversionary forces and facilities).
Officially there are no Naval Spetsnaz units in any of the former Soviet states. They are all officially referred to by cover-up false names and numbers called "Legends", which the Spetsnaz men must tell to everybody (including the Defence Minister). For example, Naval Spetsnaz Unit in the Baltic Fleet officially is Аварийно-спасательная школа №49 (Avariyno-spasatel'naya shkola nomer 49; Diving Rescue School No. 49).
All Russian military units have their own five-digit number. For example, Naval Spetsnaz Unit in the Baltic Fleet officially has military unit No. 10617.
Among Navy and Army Intelligence servicemen, all Naval Spetsnaz units are called ОМРП (OMRP, Отдельный морской разведывательный пункт (Otdel'nyy morskoy razvedyvatel'nyy punkt; Detached Naval Reconnaissance Point) and have their own numbers. For example, Naval Spetsnaz Unit in the Baltic Fleet officially is the 561st OMRP.
The Naval Spetznaz units have unofficial names. For example, Naval Spetsnaz Unit in the Baltic Fleet are unofficially called by themselves and other servicemen Парусники (Parusniki; Sailing Ships), because this unit is based in a settlement called Парусное (Parusnoye), whose name means "sail place". But in the 1950s and 1960s they were unofficially called Потехинцы (Potekhintsy) after their first commander Colonel Potekhin.
This link gives details of training and actions of Soviet combat frogmen, and says that they are in a KGB OSNAZ organization called Delfin. Other sources mention units called "Dolfin", "Omega", "Barrakuda", etc. But another source says that Naval Spetsnaz units have never used such names, but that the name Delfin arose as false information sold to a journalist, who published information about this unit in the early 1990s.
Every PDSS unit has around 50-60 combat swimmers. There are PDSS units in all major Naval Bases.
Every OMRP has about 120-200 combat divers. There are now four OMRP's in Russia, one for each Fleet: Northern Fleet, Baltic Fleet, Black Sea Fleet, Pacific Ocean Fleet.