General weather conditions: 

The country's climate includes mild winters and dry warm summers cooled by a system of seasonal breezes, popularly called "meltemi" (nothern wind) or "maistro" (NW wind). An outstanding feature of the Greek climate is its ample sunshine which rates an average of more than 300 days per year. Average daytime temperatures in Athens in springtime range between 17-27 degrees C (65-79 F). In summertime it warms up to between 25-35 degrees C (77-95 F) and in autumn this drops to 22-27 degrees C (70-79 degrees F). Temperatures in the islands usally range at 2-5 degrees C below the prementioned values. Water temperature in the sea from June to October averages about 22 degrees C (70 degrees F).

Aegean sea weather:

 In the Aegean sea the meltemi in months July and August has an average force of 5 to 7 or even 8 on the Beaufort scale during daytime and dies in the evenings. You must be an experienced yachtsman with good navigational skills to sail in the Cyclades and Dodecanese in this time of the year.

Ionian sea weather:

The Ionian islands and the West coast of Greece have a mild, more humid climate and a lush growth of vegetation. The winds in the area are less severe than in other regions of Greece and due to that the Ionian sea forms the perfect ground for a relaxed yacht holiday with kids or a crew with little sailing experience.