The union strike began on Wednesday morning and is due to run until Friday.
Ferries in Greece are to remain tied up in port for two days after unions declared a 48-hour strike on Wednesday.
The strike began at 06:00 and will run until Friday morning, according to a statement from the Panhellenic Maritime Union (PNO).
The unions declared the strike to call for collective wage agreements and salary increases, among other demands.
A large banner at the port of Piraeus near Athens read "All out to fight, abolish laws against workers' (rights)".
The unions have also criticised poor working conditions, with few staff deployed on ferries during the COVID-19 pandemic.
The action has disrupted many ferry services across the country, with no boats travelling between the mainland and Greek islands.
Many of the country's islands are too small for airports and depend on ferries for the transport of goods, citizens, and tourists.