I would definitely avoid August and the last two weeks of July. It can get very hot and crowded as the whole of Europe
holidays about then. June would be my preference. Hydra - get in early, leave after some others have left and have a line ready at the bows to deal with a possible crossed anchor
situation, which I regret for that harbour is comonplace.
Tip: Try anchoring
for a bay-day or overnight in the bay on the North side of the virtually uninhabited island of Dhkos just North of Hydra...fantastic spot!