Sikinos is one of the least visited islands in the Cyclades, a small, steep island between Ios and Folegandros with a hilltop Chora, a handful of beaches and a harbour that sees only a few ships a week. Santorini, just to the south-east, is the opposite: the busiest island in the group, with an international airport and ferries in every direction. The crossing between them links a real escape with the main transport hub of the southern Cyclades.
Most travellers take this route at the end of a stay on Sikinos, to catch a flight from Santorini or connect to a ship bound for Piraeus or Crete. It also works the other way round, as a quiet two or three nights off the Santorini circuit. Either way, the crossing is short and cheap on the conventional boats, with fares from €6.50 when we checked.
The boats you will see here are mostly small and conventional. Fast Ferries runs the most frequent service, about 3 to 5 days a week in last year's data, and the local Maistros Santorini boat adds 2 or 3 days a week. Blue Star Ferries calls about once a week, and Seajets appeared only once a week in a couple of months last year. That makes the route reliable all year but thin on any given day, so check which companies sail on your date.
The practical challenge is at the Santorini end. All these boats arrive at Athinios, the main port under the caldera cliffs, where one road climbs to Fira and traffic builds when several ships dock together. If you have a flight, allow generous time between landing at the port and check-in, and pre-book your transfer.