Let's make it clear right from the start: there are no marinas in this area, apart from the home port, most probably Kalamaki, near Athens. A single week cruise lets you go not much south of Spetses island, while a two week cruise allows you to go as far south as Kyparissi bay, SE Peloponnisos. If on a two week tour you can alternatively sail down the first four of the western Cyclades islands (Kea, Kythnos, Serifos Sifnos) and then turn westward to the southern tip of Hydra (Hydra harbour gets extremely crowded and I would be surprised if you found a place there even on triple rafting row!). You can find very useful information in Heikell's Greek Waters Pilot book. Chartering companies I would suggest are FYLY and Nomicos Yachts, respective websites:
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Nomicos Yachts charter: Home
You will surely make sweet dreams by reading sailing logs at the following links:
Travels with S/Y Thetis
Yacht charter Greece and sailing holidays Greece and Turkey - Marine navigation - Yacht charters Greek islands and Turkish coasts
Do not hesitate to send me a personal e-mail asking for specific information or clarifications.