Well... I am not sure if you are able to sail (sometimes there's not wind enough), but rowing for sure (I did it and loved it!). I believe it is possible to rent a boat with motor and maybe there is even organized trips. You may start by checking this out: http://www.yachtcharter.fi/reitit_uk.html.
I hope it is a good start.
Greetings from Barbara. about 6 years ago
Not sure about the organized tours, I will check that later. But I just have to say that the lake Saimaa - the largest in Finland - is absolutely the best lake to sail in. It has many legendary places - like Punkaharju, Savonlinna, Imatra (the rapids is a must see), Lappeenranta with its unique fortress, Puumala etc - and most Finns consider it the most beautiful lake in Finland if not in the world. Although we my be partial. : )
Also lake Päijänne and in the north the lake Inari are good choices.
If it can be something else than lake, Turku archipelago is just perfect place to sail in. And there is definitely organized tours in summer months. My in-laws have their own sailboat and I have been there many times (and I have lived by the lake Saimaa all my childhood, that's why I am so pro-Saimaa ; ) and I have seen many adverts about organized tours while visiting different harbors and islands and famous sites. Well, not too many but there are companies so there are enough choices. More details later. about 4 years ago