It's now a really well done museum, and you get out to it on a small boat that floats you into the Baltic from the main harbor.
Fans of naval history and maritime museums will be particularly entranced, but the setting is lovely and the museum so interesting that even disinterested family, travel partners or other associated hangers-on will find it worthwhile. This year we only had time to float between the six islands on a boat, but we visited one Eastertime a few years ago and the exhibits are apparently still as fascinating.
While the sights are pretty and the beer nice and cold in the summertime, it's even more magical in the winter months (like, at least nine-twelfths of the year) when the little ferry goes crunching through the ice that stretches across the Baltic Sea to the horizon.
Make sure that you pop into the tall Suomenlinna church; it's well worth a few minutes in its stark white-and-pale-wood sort of way. The historical boatyard is equally fascinating. To get there: take the Suomenlinna ferry from the Kauppatori market square at the bottom of the Esplanadi promenade, 10-15 minutes' walk from Helsinki train station.
[Sea-level photos: John Walton for Jaunted. Aerial photo: Migro.]