Drug smuggling: Underwater drones seized by Spanish police - https://www.bbc.co.uk/news/world-europe-62040790

Jim Bell's comment:

I actually thought of this 30+ years ago,  unmanned, underwater smuggling drones, but I made no attempt to implement this.  At that time, GPS was very new, solar cells were weak and relatively ineffective.  

I also figured that it could be an interesting hobby, minus the drugs of course.  Whether this could get through the plastic trash, and 'ghost nets' I don't know.