So, a meaningful solution would be to have a small, but well equipped clinic on board. Required staff can be airlifted in from the mainland within minutes in emergencies - an airport helicopter transfer in NYC costs $1,600, so the cost isn't prohibitive. Regular appointments can be attended by the doctor taking the ferry.
The "avoid making arrests, and throw trouble makers out" approach seems viable. Make sure companies on the boat post a bond for their employees that will cover throwing them out (sending them to their country of citizenship or the ships flag state for trial) if they won't/can't leave on their own.
The "avoid making arrests, and throw trouble makers out" approach seems viable. Make sure companies on the boat post a bond for their employees that will cover throwing them out (sending them to their country of citizenship or the ships flag state for trial) if they won't/can't leave on their own.