Wikipedia
Narco-submarine
The first known vessels, detected in 1993, were semi-submersibles since they could not dive: most of the craft was submerged with little more than the cockpit and the exhaust gas pipes above the water. Newer narco-submarines are fully submersible, designed specifically to be difficult to detect visually or by radar, sonar and infrared systems.
Cargoes carried are typically several tons of cocaine. For example, in 2015 the largest cargo of was seized on a semi-submersible which had been tracked by aircraft.