But surely if you want to just look and sound like an ordinary boat, you don’t need to run partially submerged with waves over your deck and a low profile hull - you could just... use an ordinary boat?
A small boat of the size most governments are going to ignore struggle to move tons of drugs, or carry more than 1-2 people. Which is what these subs are doing.
Why not have a little dinghy where the operator is and then under the water, like 10ft deep (or deepers, or adjustable length) you have an underwater pod carrying the load along with supplementary motors that are tied into the dinghy's onboard motor?
As an engineer I think this would be a dream jobs to design and built low-tech yet effective solutions for smuggling.
This is in fact a common strategy. Often the “submersibles” aren’t self-propelled and are basically underwater barges trailing a fishing or leisure boat.
A few years ago there was a story on here, https://news.ycombinator.com/item?id=9143784 , I've read some more since then but I can't readily find them (it was over the span of a few years). Maybe they were re-hashes of this story, could be. I do remember reading another story about narco subs some years ago (I too find the engineering fascinating) and one described how the technical work was (at least in that story) done by poor students who were roped in bit by bit until they got in so deep that they couldn't leave, and found themselves stuck in remote South American swamps in horrible conditions. Can't find that story though, maybe someone else knows/remembers?
I don't know anything about organized crime other than what I see on TV, but not long ago there was a local story here about a couple of chemistry students who were put to work in an MDMA lab and got themselves killed because of bad ventilation. From what I can tell (again, this is from following media, I may be wrong), engineering in criminal enterprises is low status and quite dangerous, and not even paid particularly well. Cooks in drug labs get into accidents because of how rickety those labs are, there was an encrypted phone dealer around here who got himself killed when his network was dismantled by the police, the telecoms engineers in Mexico in the story above.
Surface radars don't work too well with objects very close to the sea. It becomes hard to pick them out from the clutter caused by reflections especially if their size is comparable to the height of waves. That's why floating stray shipping containers are so dangerous to small boats and why sea skimming missiles are so effective.
Because you want the best of both worlds: The profile of a small boat, and the carrying capacity of something much larger. Hence, large boat that mimicks a small ordinary boat by many measures.
I'm also sure the increase in ordinary boats (which you'd need for carrying capacity) would be noticed.
