Quite a few of the pilots in the airline I work for either have classic aircraft or cars or boats as pets and do fettle them. Quite a few also have boats, various and fool around with them, too and in some cases, they are better at maintaining them (the classic aircraft) than airline-bred engineers.
Your industry is, by it's very nature, ruled and regulated by a plethora of national and international regulatory bodies, particularly for anyone who waves even a 1.5 Allen key at the airframe.
With leisure boats, not so much. Certainly the builders of such craft are ruled and guided by international build regulations: but, once the thing leaves the factory and passes into the hands of it's first owner, it has essentially escaped into the wild. From that point on, certainly within the UK, there's no legislative requirement for any intervention by any third party.
Having said that, craft that live their lives on inland waters are obliged to be assessed so that they comply with the Boat Safety Scheme (BSS): gas supply, power supply (shore and that provided by the boat), escape provision, alarms, crew and pax safety etc. Within the BSS, there are 2 levels of certification: one for Private Hire and one for Private Craft.
. . . but for those craft that are offshore/coastal - not a thing: there is nothing to legislate for the over-monied doughnut from getting his hands on a 30'-50' craft, figuring out how to fire up the engines and away he goes. No legislative requirement for training, insurance, seaworthiness etc.
The only thing that he is obliged to have is a marine SRC-assuming the he has a radio fitted (again, no legislative requirement) and knows how to turn it on.