FCDO advice including information on permitted duration of stay and visas for Turkey: www.gov.uk/foreign-travel-advice/turkey
There are various sources for information on the latest restrictions in force due to the Coronavirus pandemic:
RYA Covid-19 hub - https://www.rya.org.uk/coronavirus including advice and information for recreational boaters at https://www.rya.org.uk/coronavirus/advice-for-boaters.
Information on entry requirements can be found at https://www.gov.uk/uk-border-control?priority-taxon=774cee22-d896-44c1-a611-e3109cce8eae
Noonsite - https://www.noonsite.com/place/turkey/
For information on the validity of RYA certificates abroad see Evidence of Competence Abroad.
Evidence of competence is required. Turkey has not adopted Resolution 40 but for a UK flagged boat an ICC (valid for the vessel concerned) is generally acceptable and is recommended.
Turkish translations of many RYA certificates and their related course syllabus are available from the RYA website. www.rya.org.uk/go/translations
The Q flag should be hoisted and entry should be made through a recognised port of entry.
Visiting yachts require a transit log, which must be surrendered on departure from Turkey.
The courtesy flag should be flown and should be in good condition.
See holding tanks for information on the discharge of waste water.