True, shipyard workers need vocational training, and they need to be certified for the task given to them. Ordinary construction workers would find much difficulty re-learn a new skill related to ship building. Electricians and welders need years of training and experience to be proficient, and there are many grade levels that need certification.
Most of the construction workers are from rural areas and usually from farming backgrounds. They are likely returning to their home provinces taking up farming jobs again, or do some small businesses, or get another job at new emerging developing provinces far away from coasts, or even joining Chinese construction companies in foreign countries, especially on the Belt and Road projects.
China actually is facing some recruitment problems to get enough workers in some industrial sectors, epecially blue collar workers.