Just to cover a couple bits of ground here:
Size of nuclear weapons; see:
http://www.globalsecurity.org/milita...itions/vla.htm
Size and possible availability of nuclear payloads, whether recovered or purchased:
http://www.lostsubs.com/Soviet.htm
http://www.fas.org/man/dod-101/sys/m...row/shkval.htm
"Apparently fired from standard 533mm torpedo tubes, Shkval has a range of about 7,500 yards. The weapon clears the tube at fifty knots, upon which its rocket fires, propelling the missile through the water at 360 kph [about 100 m/sec / 230 mph / 200-knots], three or four times as fast as conventional torpedoes. The solid-rocket propelled "torpedo" achieves high speeds by producing a high-pressure stream of bubbles from its nose and skin, which coats the torpedo in a thin layer of gas and forms a local "envelope" of supercavitating bubbles. Carrying a tactical nuclear warhead initiated by a timer, it would destroy the hostile submarine and the torpedo it fired. The Shkval high-speed underwater missile is guided by an auto-pilot rather than by a homing head as on most torpedoes."
Just something to ponder.