In front coastline of Helsinki you can have a cruise with local ferry companies. We paid a visit to the old Sveaborg castle, the "Suomenlinna fortress".
A short list of places that can be seen during the cruise
One of the most often photographed sights in Suomenlinna fortress
Suomenlinna fortress has some places that are not easily reachable
Kustaanmiekka strait in Suomenlinna fortress