Crew members need certificates, not just people who want to work on ships.
The internship period of ordinary crew members (mechanic sailors) is six months, and they usually get service fees ranging from several hundred to one thousand during the ship. When the internship expires, it is a certificate of competency or a full crew member. The internship period of senior crew (three officials and three tubes) is one year.
Cooks and electricians don't need certificates of competency, so there is no internship problem. They get paid when they board the ship, but the salary is lower than that of qualified people.