Scuba diving
Divers face dangers that can hurt or kill them if they don't know what to do.  They can use up all their air, or get decompression sickness or nitrogen narcosis.  A person must be trained to use the equipment and dive safely. When they show that they can dive safely they get a certification card.  The biggest organization for certifying divers is PADI - Professional Association of Diving Instructors - but there are many others, depending on the country.  Some tourist places have a short course without certification and then the instructor will lead the diver in a shallow dive, all in one day.