Entry into Greece requires vaccination or PCR test.
Entry to Thassos per Sea is written:
“In order to board the ferry to Thassos, you will need to present your Covid documents (proof of vaccination, proof of negative PCR…” Ther is no OR between vaccination and negative PCR. Do I really have to show a negative PCR test every time I use the ferry?
Greetings ZAV
Hi zav 
No. If you are vaccinated then you don’t need any test of any kind to use the ferry.
Thank you for asking! We will clarify it here:
Hi, we just got here yesterday and are so happy that it really happened. To get on the ferry, they handed us a form to fill in (you hand it in once on the ferry), but nobody asked to see test-results or proof of vaccination.
Welcome back to Thassos, hobgoblin! 
It appears the ferry companies are using the honor system. Every passenger must complete the form and make all necessary declarations without scrutiny from the ferry companies or port authorities.
In case anybody wants to see the form in advance, here’s a scanned copy of the form (PDF format) I just picked up from the ferry port.
Ferry Form.pdf (1.8 MB)