Does anyone know a reliable site to book Greek island ferries online?

I've never found that you need to book. I have always bought my tickets up to half an hour before the ferry has left. The only occasion that I would ever book, is for the tiny hydrofoils that have only a capacity of about 50, and you would only need to in the very height of the summer. You are talking about the big massive car ferries, so I sincerely doubt of there being any need to book. However when it comes to the timetables.... You would need to look up the company of the ferry which operates in those areas. I only know the ferries for the Dodecanese. I suggest you google the place...Santorini, and then ferry companies. And when it comes to Athens, search for Pireaus, which is the port the ferry would leave from. Check out lonely planet's website. They may have info on ferry websites under the Greek islands. Good luck, sorry I couldnt be more help.

As for no need to book, sometimes you need to book in advance, if it is a high peak season, weekend, etc. or if you take the fast ferries, and definitely if you have a car. You don't really need to book so much in advance, but check www.ferries.gr Just make sure you know what ferry to choose, because there are fast new ones and slow old ones.

Try www.openseas.gr! I never book tickets in advance either just as the previous contributor said. Very often, due to bad weather, the ferries are cancelled and it is a real headache to get refunded! If you fly into Athens, I would recommend you to fly to Santorini! Have a nice vacation!

http://www.greecetravel.com/dolphinhellas/dolphin_ticket.html