As of now (July 2021), many countries require a negative COVID test before boarding your flight back home from Greece.  For US guests, the CDC states: "Passengers must be tested with a viral test that could be either an antigen test or a nucleic acid amplification test (NAAT)."   You can obtain a rapid antigen test at this testing center near Zea Marina when you return on Friday. You will receive results in less than an hour. You should be able to make an appointment ahead of time as well, and the captain will be able to assist you if needed.