Hydra
 

I am planning a trip to the Island of Hydra at the end of October and have the following questions:

Will most of the restaurants and cafes still be open?
Is there daily fast boat service to and from Pireaus?
I know that it depends on the the weather, but is the island usually deserted at this time of year?

I would suggest that you bring your message to the top (by adding another message that does not have to say anything more than "ttt") and hope that Brotherleelove sees it. He is the principal Greece expert on this board.

ttt

Some restaurants/cafes will be open.
Yes.
No, greeks live on the island.

you will love hydra. i wrote a report on my athens trip ages ago.

unless you like jet motor noise and to sit INSIDE and not realy see anything, the fast ferry is not as soothing as the slow ferry.
i took both and would opt for the slow both ways unless i was really in a hurry or the weather was bad.

it was very pleasant to sit outside on deck, although the diesel steamer also makes noise.

i enjoyed the side trip to beasti beach on the tip of hydra.

i only spent two days and one night so i couldn't explore as much as you will be able to.

We were there for three nights in June. We really enjoyed walking around the island and I imagine that would be very pleasant in cooler weather.
You may also avoid all of the crowds. I'm just guessing but the restaurants where the locals eat would be open and some of the "touristy" restaurants around the harbor might be closed....not a bad thing at all.
Have a wonderful trip.