There is a new publication that appears to be the definitive source on Narco subs and related craft including semi-submersibles and un-manned, towed narco “torpedoes.” It certainly has the best selection of photographs I have ever seen on the subject.
Narco-Submarines Specially Fabricated Vessels Used For Drug Smuggling Purposes
In cooperation with InSight Crime & Small Wars Journal— El Centro
Byron Ramirez and Robert J. Bunker, Editors
And of course these could be used for smuggling in things other than drugs.
You can access the report in pdf form here.