There exist entire bodies of knowledge about ensuring that equipment meets the physical requirements of the intended use, that the systems are designed to fail in a safe(r) mode and have redundancies in case of failures, that processes are created to operate more accident-free and also recover from accidents, and the ability to manage and mitigate risk in general. These come from centuries of seafaring and aeronautics and decades of spacefaring.
It is all there, and just because you don't know about it and I cannot recite it all off the top of my head does not mean that it doesn't exist and that it could not be implemented. Regulators know how to find the right people to evaluate the risks and set suitable requirements. (That said, in some industries, regulatory capture and corruption is a real problem, but it does not seem to be the case here.)
I didn’t mean to suggest that there are not regulations they could have used, I’m more interested in how the other commenter thinks ‘expertise’ should be judged, especially from the point of view of the passengers.
As someone who has dealt with certification bodies for designs critical to their operators lives I am well aware of the processes involved. In my experience regulators are often the least qualified to determine risks, since the most qualified people make more money on the other side of the fence. So regulators depend on following a rulebook rather than expert judgement. There are definitely rulebooks for submarines, but they are generally for naval applications with billions of resources, not to minisub expeditions.
The company was open that their design was cheap and cheerful and followed no certification or standard. The riders knew this and could have chosen more expensive, safer alternatives. But did they truly ‘know’ how much extra risk they were taking? I don’t think there’s any good way to judge that.
It is all there, and just because you don't know about it and I cannot recite it all off the top of my head does not mean that it doesn't exist and that it could not be implemented. Regulators know how to find the right people to evaluate the risks and set suitable requirements. (That said, in some industries, regulatory capture and corruption is a real problem, but it does not seem to be the case here.)