Working as a seaman requires holding corresponding seaman certificates, and the social population certainly does not have these certificates, so it will require study and research, which will generate expenses. Then the other purposes mentioned above are obviously for profit.
These so-called shipping companies are actually intermediaries. Whether it is a regular shipping company or a liar, there are deceptive information when releasing this information. Liars don't say much just to cheat money. Intermediaries often exaggerate seafarers' wages and advantages, and will arrange your work after sending you to school and getting a certificate. But the cost of pre-training and study will directly exceed the cost of going to school.
If you want to be a sailor, I suggest you find out the real situation of seafarers yourself, and then find a short-term training course for seafarers in a university. After getting the certificate, you can go to an intermediary to recommend a job, but most schools will recommend a job. This will reduce the cost by more than half.