I don’t even know how to go about enumerating who those people are, their ships, or where they are anchored. With that information a well-organized and/or funded group could at least get someone out to these people to check on them, provide basic supplies, and perhaps some form of reliable communications.
A lot of problems seem insurmountable large and complex, and even this one seems so if your goal is to free these people of their legal liabilities - but if you set aside trying to solve the reason they’re stuck onboard these ships in the first place, providing basic humanitarian aid to them seems doable.
ETA: This looks like a good place to start - https://www.ilo.org/dyn/seafarers/seafarersbrowse.home