Small 6 feet long submarines which just head to a predetermined gps coordinate and travel at 1 mph, powered by a solar panel floating a few inches under the water surface would be very hard to detect without very specialist tech. They could probably be built for $1000 each without any custom built parts - simple RC plane servos, a compass and GPS, and an arduino should suffice.
The smugglers could then head out at their leisure and pick them out of the water after their 2/3 month transit time.