The ferry service is operated by the Helsinki Region Transport Authority (HSL). Depending on the season and the time of day, ferries run 1 to 4 times per hour from the morning until the night throughout the year. Please see the detailed ferry schedules here (pdf).
The ferry departs from the east side of the Market Square, opposite the Presidential Palace. In Suomenlinna, the ferry arrives at and departs from the main pier of Iso Mustasaari island, on the north shore of Suomenlinna.
The ferry is part of the city transport network and all HSL tickets are accepted. Tickets need to be purchased before boarding from the ticket machine on the departure pier. In the summer, tickets are also sold at the ticket booth in the Market Square ferry terminal. Tickets are not sold on board. Please see the HSL website for ticket prices. The crossing is free with a Helsinki Card.
The ferry has space for 1-2 vehicles. Reservations for vehicles should be made with Suomenlinnan Liikenne Oy.