Among our first client bookings was a short visit to Santorini and since that day the subject of ferries comes up when discussing the Greek Islands.
Greece does indeed have a reasonable ferry schedule, making it seem as though visitors can just jump on any ship at any time and enjoy a pleasant change of scenery while exploring any number of unique island destinations, such as Mykonos or Zakynthos...
For environmental reasons, I really hate to advocate flights over ferries, but the reality is that your trip to Greece is probably very limited in terms of time - you may only be spending a few days in each destination or a week or so in the country altogether.
That means your time is not only valuable, but its management is critical to the success of your trip.
Ferries only run once or twice a day, so you don't have many options in the first place and if you're misconnected for any reason you don't get a chance to recover on the same day with a later departure.
Even the fast ferries aren't that quick
Trying to make the most of your time won't happen when you spend nearly a full day travelling by sea between islands. Expect 8 hours of actual travel time from Athens to Santorini, for example! That's pretty much a full day of your trip without taking into consideration any advanced arrival process or delay;
Rough seas and other delays
Like any other vessel, the ferries can experience delays and may be down-right unreliable when the weather isn't playing the game. Imagine your 8-hour journey to Santorini is delayed from even departing by several hours or cancelled completely. Not many options to recover and you risk missing out on a whole island if you have to reroute.