As of today, Greece allows Pleasure & Commercial Yachts (of any flag) to enter coming from any port with passengers and crew on board.

Specifically, vessels can also enter from Turkey & Albania in the following ports but with crew* only:
Turkey: Kavala, Mytelene, Chios, Samos, Kos & Rhodes
Albania: Corfu, Igoumenitsa & Preveza

(All crew must either be vaccinated or present a negative PCR test)* Anyone of the following nationalities can fly to Greece or arrive as guest on board:
European Union Countries, Australia, North Macedonia, the United Arab Emirates, The United States, Britain, Israel, Canada, Belarus, Bahrein, New Zealand, South Korea, Qatar, China, Kuweit, Ukraine, Rwanda, Russia, Saudi Arabia, Serbia, Singapore, Thailand & Schengen Countries (Persons from other countries with residence permits in the EU or spouses of EU citizens are also allowed).

Passengers from those countries are allowed to enter, as long as they prove that they have received two doses of a COVID-19 vaccine 14days before their travel or show a negative PCR test carried out 72 hours prior to their arrival.
All passengers flying to Greece must fill out the digital PLF form at least 24hrs prior to boarding their flight.