We find the most enjoyable times for charter sailing in Greece are in May/June as well as September/early October. The weather is generally dry as well as nice and warm between June and October (there is a small increase in rainy days in May, Sept, and October). The Aegean tends to warm up during the season with cooler water temperatures in May (around 68 degrees F) and the warmest water temperatures in September (around 82 degrees F). The main downside to July and August are the overwhelming crowds as well as the risk of a possible heat wave.
In the end, it comes down to personal preference and the weather itself can change from week to week, so there are never any guarantees.