Warning! The SOL cache series expose the geocacher to excessive UV radiation in an environment where it feels comfortably cool. Do not get a sunburnt, since it raises the risk of skin cancer significantly. There's nowadays also mites in the Helsinki archipelago that are contaminated with the TBE virus. The proper means to hedge against the disease (Kumlingen tauti in finnish) is the vaccination.
The round trip ferriage from Eteläsatama to Valkosaari is 3.5 euros. There is heavy passanger ship traffic in winter, which means that the ice route to the island is most likely dangerously unevenly frozen or just unfrozen and therefore not accessible.
Observe that there are two islands. One of the buildings out there appears in Mika Waltari's novel Appelsiininsiemen (1931), though I'm not sure which one. Should you know it, please email me.
Take the camera with you, when you come in here. I'm pretty sure you agree that the angle to the Helsinki city centre is just splendid.