Naxos and Sifnos sit on opposite sides of the Cyclades, about 42 nautical miles apart, and this crossing is how island-hoppers swap the busy central islands for the quieter west. You leave the largest island of the group, with its long sandy beaches and marble villages, and land on a smaller, walkable one known across Greece for its cooking, its pottery and its paths between hilltop villages.
In this direction it is a Seajets route and nothing else. The high-speed boats usually call at other islands on the way, which is why the trip time swings from just over two hours to more than four and a half. Look at the arrival time before you pay, not only the departure: two sailings on the same week can differ by a couple of hours.
The other thing to know is the price. At €94.70 for an adult at the cheapest, this is an expensive hop by Cyclades standards, and there is no cheap conventional ship heading this way to fall back on. Two people pay €189.40 one way at the lowest fare. If that is too much, a two-leg trip through another island can cost less, though it takes longer.
The season is the limit. Direct boats run only from March to November, busiest in July and August, when the daily departure can sell out. Outside that window, plan on travelling via Paros, Milos or Piraeus instead, and give yourself time for the connection.
