Last time I looked we are still in Europe, so it would be an Interrail pass.

However Greece is not renowned for train services across water to the islands, nor internal train services. The train passes do not include ferries, but may give discounts. The ferry pass only include a couple of port trains.

Do some research on ferries - IR do a Greek Ferry pass but it has complex rules and only covers 2 ferry companies.

Search the community for more info on Greece and Ferries.