Interesting idea. Compared to a submarine yes, but what this would more likely be up against, from the perspective of cost is a panga boat:

http://www.independent.com/news/2012/jul/26/13-people-arrest...

"The push out to sea is spurred by increased crackdowns on the border, and pangas loaded with multiple fuel barrels have been spotted as far north as San Francisco. The Los Angeles and Long Beach areas have seen a similar increase in incidents."

Lets see, to ship ~$6 million worth of pot paying someone a small fraction to drive a boat up the coast, plus a few thousand for a boat, I'm not seeing a reason why they'd want to make a drone sub to do this.