They are demanding back pay from shipowners, a collective wage deal and an end to the use of undocumented and uninsured workers.
The Panhellenic Seamen's Federation (PNO) also wants the government to scrap plans to regulate minimum crew numbers, saying that would lead to redundancies.
"The law wipes out the seamen's profession and the rules underpinning it," the PNO said.
The strike has hit Greek islands who rely on the ferries for supplies and ministers may force seamen back to work using the same power they used against Athens underground staff.
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Farmers briefly disrupted motorway traffic today with sit-ins and gave free rice to drivers, in a protest against tax rises.
"We have no choice, we're on the brink of desperation," one farmer said.
Greece's latest austerity measures slash tax refunds and fuel subsidies for farmers while increasing their social security contributions.