Recent headlines indicate that the Coast guard is seizing a lot of cocaine lately. Last year, two billion dollars worth of cocaine was taken from international waters - and actually found in submarines. Thousands of pounds of cocaine have been found inside submarines mostly off of the coasts of Central and South America.
Though most subs are typically expensive and high-tech, these particular ones are not as advanced and extremely dangerous to operate. They tend to be partially submerged, with only the cockpit and an exhaust pipe visible above the water, making them difficult to detect.