Serifos and Santorini sit on opposite sides of the Cyclades, one in the rugged western chain and the other at the southern edge, so this is a longer crossing than the neighbouring hops around them. It suits travellers who have spent a few quiet days on Serifos, with its hilltop Chora and sandy bays, and want to finish their trip with the drama of the Santorini caldera, or who need to get to Santorini's airport for a flight home.
Unusually for a route of this length, there is a direct boat on most weeks of the year. Ferryhopper lists sailings in every month, on two to seven days a week, shared between Fast Ferries and Seajets. The two could hardly be more different. The Fast Ferries conventional ship is the budget option, from €16.50, but its journey time varies from around 3 hr 45 min to more than 6 hours depending on the itinerary. Seajets is quick and consistent, at 3 hr 15 min to 3 hr 25 min, but it was listed from €73.70 and often higher.
That gap shapes the decision. If you are on a budget and in no hurry, the long conventional trip is excellent value and gives you a slow cruise through the islands. If you have a flight or a dinner booking in Santorini, the high-speed boat buys you certainty and several hours of your day. Look at the duration shown for your exact sailing before you commit, because the conventional times change a lot from month to month.
Remember that you will arrive at Athinios, Santorini's main port at the foot of the caldera cliffs, where transport can be stretched when several ships dock together. Sort out your transfer before you leave Serifos.