The crash took place along the Miessaarenselkä ridge off the Haukilahti coast, a favourite destination of recreational boaters.
Divers freed the pilot from the partly submerged plane, but were unable to revive him. There were no passengers on board.
Maritime Rescue says the plane was en route from Tampere to the capital city region.
The first boaters to the site called the Maritime Search and Rescue Centre, who responded by sending a number of rescue teams, including the Boarder Guard’s maritime rescue helicopter and rescue units, rescue service units and the police.