I just spoke with one of my bosses about this subject a few minutes ago. Interestingly, he’s been down this road before, when he was a marine inspector in Miami. Anyway, he tells me that in addition to the fully and semi-submersibels they inspected that were intended for commercial use, they had some that were built for strictly recreational use and there wasn’t anything the government did or could do beyond ensuring the sub met basic safety requirements for a vessel its size. These recreational subs were/are not operating in US waters; they were destined for other countries. However, the point remains that there’s nothing stopping someone from doing it and operating them here in the US.