I don't work in the maritime industry so I might easily be wrong, but my understanding is that these rules are generally enforced not via national laws directly but via insurance. You can build any kind of ship you like, but then you have these classification societies that verify whether a ship has been built to standards, and if not they won't certify it and you'll have a very hard time getting insurance for your vessel. And in the end national laws tend to say things like your ship needs to be insured etc., not directly how it must be constructed.