Most people do a visa run for few days to
turkey if in the Aegean or itali if in the
Ionian.
"There are no rules on how long a
boat must remain outside of EU waters before returning for another 18 month Transit Log. Evidence of having left the EU (a marina receipt for example) will probably be required. If the
boat is 'laid up', which usually means placed in
customs bond, the 18 month (or 24 month) clock stops until the boat is put back in use. The TPP will also have to be paid (see the cruising tax section).
Greece resumed these EU-wide temporary importation rules on 1st October 2015 and scrapped the 6 month in 12 month rules."
https://www.theca.org.uk/news/greektax