What's the feasibility of special forces insertion by sub?
1. Feasibility is high. But more than just insertion, NATO have used submarines to monitor pirate activity off the coast of Somalia and and <delete country> have used submarines to monitor pirate activity off <delete location monitored to avoid offending observed country>, to vector a naval response. In the case of NATO, after the submarine was done with the intelligence gathering on a known pirate base, the navy went ashore and destroyed the pirate skiffs. These were extremely successful counter-pirate operations due to the quality of information gathered by these submarine crews.
2. There is a whole range of special forces missions (along with very high end and niche capabilities maintained by Australian, American, French, German, Italian, and Israeli frogmen), in the real world where submarines are used for conventional and unconventional operations. Diver-propulsion devices come in difference sizes and
training to use it is not difficult, compared to combat diver qualification. Divers can be used for beach and harbour reconnaissance — in the case of Vietnam, they put divers in the water against Chinese oil rigs, which prompted withdrawal.
3. Shayetet-13, have acquired Italian-built
Deep Guardian Shelter, to insert combat divers into Gaza and Lebanon, for numerous operations that resulted in the deaths of terrorists (see:
H I Sutton - Covert Shores). But there is risk and in Sep 1997, a force of 16 from Shayetet-13 was ambushed by fighters from local militant groups Hezbollah and Amal while on a mission deep in Lebanon. 11 of the commandos were killed in the firefight that ensued, including force commander Lt. Col. Yossi Korakin, as well as a doctor from a rescue force dispatched to their aid. Hezbollah revealed that they knew the Shayetet-13 insertion point in advance thanks to the interception of video footage broadcast by Israeli UAVs that were hovering over the area in the days before the mission. 20 years later, the IDF finally confirms Hezbollah was able to intercept intelligence while being transmitted from an Israeli drone, giving it advance knowledge of naval commando operation deep inside Lebanon.
Is that a realistic useful thing these days?
4. Yes it is realistic and useful. In Mar 2015, Maj. Gen. James S. Hartsell went to observe diver insertion training between US and Singapore, during Exercise Sandfisher, where combat divers were training to operate in the littorals.
5. I have spoken to Buds and other combat diver qualified operators to help me understand Australian, Singaporean, Taiwanese and American underwater capabilities. Read up on the
US Navy’s SEAL Mini Sub — the SDV, for their conventional operations capability. Hope this helps.