Your best choice would be to fly, forget about boats, they take time and not worth the trip. easyjet, Vueling and others fly from the Mediterranean coast airport to Athens.
You can book yourselves for a cruise, taking you to different islands in Greece, also visit to Turkish coastline places. Of course, a cruise is a cruise, you will spend most of yr time at sea, and getting to know various towns limit you for a daytrip, which extra cost, or you can venture on your own by taxi.
Unless the journey is a major part of the holiday, then air is the only way. It's 2000km by air, 2500km by sea or rail - if you can easily organise sea or rail transport between the two countries.
as masBenidorm mentioned vueling has tickets from 60 Euro (one way) using train from Spain till Italy and then a ferry to Greece it will cost you much more than the flight and about 2 days of traveling instead of some hours flight.
Thank you all for your comments :)
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