For many visitors, this is the last sea journey of a Greek island holiday: the long run from Santorini's caldera back to Piraeus, the port of Athens. It is also the route people take when they want to see the Aegean from deck level on the way north.
You leave from Athinios, Santorini's main ferry port on the west coast, and you arrive at Piraeus, the main port of Athens. Athinios is small and its access road gets congested in summer; Piraeus is large, with many gates along the waterfront, and metro Line 1 runs from Piraeus station into central Athens.
The ferry makes the most sense if you are heading to Athens itself, want to break the journey at another island, or are travelling with a car. Many sailings call at other islands on the way, so check the itinerary on the booking page; if you would rather stop for a night or two, you can book two separate legs, for example Santorini to Naxos and then Naxos to Piraeus.
Before booking, look carefully at the duration of each sailing. The difference between the fastest and slowest ships on this route is several hours, and the cheapest ticket is not always on the slowest boat. If you have a flight from Athens the same day, choose an early, faster sailing and leave a generous margin, because the port-to-airport transfer adds about an hour.