Drug lords have been using homemade subs to ferry cocaine to the US from Columbia for a few years but the machines are getting more sophisticated, sometimes employing remote control systems.
The subs aren't really subs, they always have a portion of the ship above water. But at less than 18 inches of height above sea level, they're hard to detect using radar, despite being anywhere from 40-80 feet, carrying from 4-12 tons of the white stuff and 4 people. (Or none if being remote controlled!) They are reported to go slow and steady using diesel engines and up to 1500 gallons of fuel. They can make the trip at 6-12 knots in a single tank. As if they could refuel anywhere. The ships take over a year to make from fiberglass/wood or steel, and cost $2m. Not a whole lot of money since one load can take almost $200m of cargo from Ecuador to San Diego.
Here's a worrying thought:
"...if you can move that much cocaine, what else can you put in that semi-submersible. Can you put a weapon of mass destruction in it?" Navy Adm. Jim Stavridis, Commander, U.S. Southern Command