Well, we usually book in advance in Greece for any boat trip we make so I would advice you to book at least a month earlier. I should also remind you that even though September and October are not considered full season in Greece, there are lots of tourists roaming the country and especially the islands since the weather is still very hot in these months. I don’t find it impossible that you find tickets the day that you want to travel but it would be safer for you to book in advance. Almost all travel agencies in Greece can book you tickets for the islands, I am not sure how you can book online though because I have never tried it.
Alternatively to Hellenic Seaways (more expensive but faster): Seajets. Read the FAQ too.
My personal experience (native Greek) would also suggest that there is no need to book in advance. Only if, you keep checking the prices online a few a times a month until (say) May and you observe any significant price drop so you may save, otherwise prices should more or less be fixed.
Have a safe trip and great holidays!
You do not say it in your question, do you have a car?
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First, for the close experience, I went during August to Greece and did not book ahead (we were pedestrians). On a very large car ferry between Athens and some islands, booking the day before let us without reserved seats, but cabins with beds were still available. We went for non-reserved seats and it was less comfortable than the reserved ones for sleeping, but these are comfortable for sitting even on long distances (Santorini to Mykonos seems to take 3 hours). The seats are first-come, first-serve but even in August they probably do not sell out. So if you are pedestrians, that could be fine.
So it was not full but booking ahead would have been smarter, especially because buying tickets there made us lose time (the harbour is 1 hour from Athens center) and made our plans risky (if there were no more seats, our itinerary would have been messed up).
We took other more local boats and we never had any issues, but these were intended to be booked last-minute anyway.
As you said in comments, websites selling tickets do not seem particularly trustworthy, that’s a good argument. But when I went to the harbour in Athens, there was no distinguishable “official” ferry ticket seller for a carrier, only a string of many travel agency offices selling tickets for any ferry operator. And the one I bought my ticket from made a typo when copying my name from my ID (I don’t have a “greek-friendly” name, though).
On the website of Hellenic Seaways they do not mention any office where they would sell their tickets, but a forum thread suggests the travel agent Pelican Travel is the agent that is “official” reseller of Hellenic Seaways on Santorini island.
I found an article on tripadvisor on the topic that mentions a couple websites to book from. I could not find reviews for these though.
But overall, it is this kind of question that depends mostly on your prioritization of constraints: do you have a car? Do you have a tight schedule?
Notes:
To summarise, as large ferries use this itinerary, if you are a pedestrian, you can still take the risk of booking last-minute and you will have a spot. If you have a car, you take more risks.
But anyway it depends mostly on you and on how flexible you are: if you really cannot plan ahead what schedule you would want, then wait for the last minute with the risk of staying one more day. If you have a tight schedule and prefer peace of mind, book ahead.
Regarding where to book, I was not inspired by websites to buy tickets, but neither was I inspired by the travel agents in the harbour. I think there is no better choice, but if you are concerned about the security of buying online, this is totally understandable and a good point.
Credit:stackoverflow.com‘