The case in point is what you said: "Both caves and shipwrecks are allowed with Open Water".

As a rule, they absolutely aren't.

In this specific place, PADI allows some of them to make a supervised dive at a certain site following a specially installed line. This doesn't change the rule, which is "Open Water Diver certification does not qualify you to dive on shipwrecks, nor caves".

At some other place, there may be an option for a person with Open Water certificate to drive a semi-trailer truck, while under supervision from a trained professional, and over a course specifically crafted to minimize any possible collision. This doesn't change the rule, which is "Open Water Diver certification does not qualify you to drive semi-trailer trucks."