Semi submersibles are wood (older ones) or fiberglass for low radar signature, and use small sailboat engines which exhaust their gases underwater to disperse the heat signature. They have only inches of freeboard above the waterline, and pumps hose cold seawater onto carpet remnents fixed over the very well insulated engine compartments to knock down any IR signature.
They go slow (@ 5 knots) for no wake signature (big "go-fast" wakes are easy to see from the air) and have a very long range, over 1000 miles if needed. They can pass within a mile of a cutter and not be seen, and are easy and cheap to build covertly, far easier to build and operate than true submersibles, and nearly as stealthy. With GPS connected to anautopilot, they can even be sent as unmanned drones. A semisubmersible carrying a WMD could be launched at sea, and sent right into a US harbor by GPS and autopilot, or a happy Jihadist could do the navigating.
I'm not saying that anything like a semi submersible is likely to be used in this case, I just want to bring folks up to speed on what's out there in the happy world of smuggling (dope, agents or WMDs).