Shipping and Island Policy Minister Yiannis Plakiotakis, in cooperation with Deputy Minister for Civil Protection Nikos Hardalias, on Friday announced measures for ferry routes in the context of the measures against the spread of the coronavirus.
In this context, from Saturday March 21, only permanent island residents and supplies will be allowed to travel on the ferries, and those that wish to depart from the islands on the return journey.
The relevant decisions are due to be issued shortly.
In his message Plakiotakis underlined:
"As you are aware the government is taking all the measures that can contribute to the containment of the coronavirus pandemic. The main message is "we stay at home" and limit our movements to those that are absolutely necessary, in order to protect ourselves and our compatriots. In this context, from March 21 at 06:00, only the permanent residents of the islands will be allowed to travel with the ferries. All sea routes will be conducted for the permanent residents and for absolutely necessary reasons and to supply the islands, as well as for those that want to leave the islands. In this way, we protect the islanders because, as the prime minister said in his message, the smaller the island, the more difficult if impossible will be the treatment of someone who gets infected by the virus. We therefore show social responsibility. We prevent the spread of the pandemic. We stay home," Plakiotakis said.

Tax return as proof of residence

Sources at the Shipping and Island Policy Ministry on Friday late afternoon clarified a key point concerning the ferry travel restrictions to the islands announced earlier in the day.
They elaborated that since the only passengers now allowed to travel to islands are their permanent residents, the only acceptable way to prove this will be for the passenger to show their tax return form, effective as of Saturday. 
Meanwhile, sources added, ferry companies will try to connect their systems with Taxisnet - Greece’s online tax services - to quickly confirm a passenger’s claim. 
The travel restriction, going into effect on March 21, also exempts suppliers of goods to islands and people already on islands who wish to return to the mainland. 

See also:

Mandatory 14-day quarantine for passengers from abroad in effect from Friday noon