FRS Finland Oy operates the ferry or the waterbus from Helsinki Market Square (Kauppatori, the pier is called Keisarinluodonlaituri) to Vallisaari Island. It takes about 25 minutes. In July and August they operate every day (once in an hour) and in September on Fridays, Saturdays and Sundays. A round-trip ticket for adults costs 13.90 euros. You can buy tickets from the ticket office (a small kiosk) next to the pier. Notice that in Vallisaari the arrival is at Luotsipiha Pier (Pilotyard Jetty) but the departure is from Torpedolahti Pier (Torpedo Bay Jetty). You can hop on any return transport. Usually free seats are always available (only the time of Helsinki Biennial may be an exception, next time in 2025). So you don't need to know your return time in advance. Take your time in Vallisaari. There is very much to see and do there: historical sights, beautiful nature, sightseeing platforms with wonderful sea views, a picnic ground and excellent cafeterias. Read separate Tripadvisor reviews about Vallisaari for more information. And by the way, I appreciate that FRS Finland Oy has been awarded the Sustainable Travel Finland certificate and they are committed to developing sustainable tourism.