> A hobby can mean still serious bussiness if you find the right buyer (Think in minecraft for example).
Building a submarine is not that kind of a hobby.
> Casually China has the strongest and biggest (ever-expanding) aquaculture sector in the entire planet.
Yes, so they don't have much to learn from your average hobbyist sub builder.
> Militar technology is totally out of reach for they, too big, and uncloneable, but I bet that a lot of companies with fresh money to burn could find good uses for a toy like this.
https://en.wikipedia.org/wiki/Type_093_submarine
How is that for a toy? That's a nuclear powered sub the Chinese have a bunch of them, this one displaces 7000 tonnes submerged compared to the 40 tons of the Nautilus, has a crush depth orders of magnitude deeper than the Nautilus and carries a bunch of cruise missiles.
Do you honestly believe the Chinese could learn even a single useful thing from Madsen's toy?
The only people with a direct interest in running private submarines are the drug cartels and some salvage companies, and neither of those would lead to Madsen killing a journalist.