Ghalib Kabir, on October 28, 2021, invited my assessment on advances in 1. AUVs/UUVs 2. Small Subs (up to 600 tons) and 3. Minisubs (up to 300 tons). The opaque and secret nature of all these undersea machines means my assessment is subjective .
As an aside Ghalib’s request was just in time, because I’m suffering interminable AUKUS nuclear sub (aka Aus-SSN) writing numbness.
My necessarily subjective assessments are:
1. The US has been ahead with large AUVs/UUVs for more than a decade - particularly with the Project Orca (was Echo Voyager) and see my 2016 article.
Russia's huge Poseidon, aka Status-6, nuclear warhead torpedo UUV "drone" remains highly secret technology. So, I can't assess how advanced it is.
2. The West Germans led the way in Small Coastal Submarines (up to 600 tons) with the Type 205, 206 and 207/Kobben. Iran’s 600 ton Fateh-class seems to have been developed from, or been inspired by, those West German designed small subs.
3. The Italians led the way with diver delivery vehicles (from WWII) and later the Italian Cosmos minisubs.
North Korea's (Yono-class), South Korea's (Dolgorae-class), Pakistan's (MG-110) and Iran's (Ghadir-class) seem to have been developed from, or been inspired by, Italian Cosmos technology.
The Indian indigenous coastal submarine (only at DESIGN stage, with a link provided by Ghalib to H I Sutton's Covert Shores artwork here) may well be a development of the Soviet/Russian "Piranha" (NATO designation Losos-class) small-mini submarine displacing 390 tons submerged. But then again is may owe something to the West German designed Small Coastal Subs (the 205s, 206s and 207s mentioned, with links, above).