It takes only 15 minutes to ge to Suomenlinna from the Market Square!
Helsinki City Transportation: Ferry Traffic
It is possible to visit Suomenlinna year round with the Helsinki City Transportation ferries, which run daily from the Market Square. During the wintertime the ferries leave from the Market Square once an hour and during the summertime more frequently. Tickets can be purchased from the Helsinki City Transportation service points or from a vending machine at the Market Square. The ferryride takes only 15 minutes.
You can find the ferry timetables and the ticket prices here.
Island hopping: Suomenlinna, Lonna ja Vallisaari
Suomenlinna can be reached from the first of May to the mid-September by a JT-Line water bus from the Market Square. Tickets can be purchased at the water bus or reserved in advance for groups from the Ehrensvärd Society.
With JT-Lines you can visit three different islands; Suomenlinna, Lonna and Vallisaari. The ferry ride itself is already a wonderful experience!
You can find the timetables for the waterbus here.