Anafi is the last stop in the south-east corner of the Cyclades, a small, quiet island a short way east of Santorini. For most visitors, this crossing is the link back to the wider world: Santorini has the nearest airport (JTR) and far more onward boats, so the Anafi to Santorini ferry is how an Anafi stay usually ends.
The two ends could hardly be more different. Anafi has a single harbour, Agios Nikolaos, on its south coast, with Chora 3 km up the hill and the beach at Klisidi about 1 km away. You leave from a small quay with a few cafés and tavernas. An hour and a bit later you are at Athinios, Santorini's main port under the caldera cliffs, 8 km from Fira and 20 km from Oia, where several ships can be unloading at once in summer.
The crossing itself is short and cheap: adult fares started at €7 when we checked Ferryhopper. The catch is frequency. Blue Star Ferries and Maistros Santorini each run this leg on only a handful of days a week, so Anafi is not an island where you can decide at breakfast to leave that afternoon. Plan your departure date around the boat, not the other way round.
If you are catching a flight out of Santorini, build in a buffer. Ferries on small-island runs can be delayed by weather, and with no daily sailing on some weeks, a missed connection is harder to fix here than on the busy central routes. Travelling on the day before your flight is the safer plan.