Some ferry routes require booking in advance, particularly during the peak season, when millions of tourists do the Greek island hopping game. This is valid for longer routes such as Athens to Mykonos, Athens to Santorini or Athens to Crete. When it comes to very short routes, there are ferries leaving every hour or so. If you want to get from Igoumenitsa to Corfu, for instance, you don’t need to buy ferry tickets online. Going from Piraeus to Heraklion, on the contrary, requires advance booking, because you may not be able to find a place to sleep if you buy your ticket the very last minute. Spending the night on a chair isn’t that great, though, so you should plan your trip properly.
Here’s a ferry timetables and routes search engine you can use to find and buy your tickets online: