You miss the point. The poor SOB ship won't even know it. It would be a submarine that attaches itself in Japan to a ship known to head to the US. Or mid-rout. And that's that. In the US, we have a small ship waiting for the cargo sending a signal to wake it up and detach.

The beauty of current AI is that the compartment can be set to notice divers nearby and "walk" to the other side of the hull. Since sonar can't pick up small attached objects, boom you got yourself the equivalent of a smuggling bed bug. The crew can be all detained while this happens because they are not in on it.

The only issue would be noise made if you have listener crews. That one would be tough to solve. That's why you attach yourself to large boats, not to small smuggle ships.