Why even go down this route if it is hard to prove? Just book a medical check up appointment in a hospital or a cooking class, that should be way easier.
The problem is, that foreigners need a letter from employer to confirm the seaman is sailing on a Thai vessel. As most ships fly a flag of convenience, this is rare. For Thais it seems to be pretty straightforward and they are quite fast issuing them. Usually the crew members go pick up the smb at the Bangkok office.
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