Thank you! :)
It think those boats aren't for rent. There aren't that many businesses offering boats to rent here in Tampere which is a shame as the lakes are really beautiful with their islands.
I have rented a rowing boat once to just row around and go to the neares island, from a place called Kurpitsatalo (pumpkin house) that locates near the south end of the tunnel, on the shore of Pyhäjärvi. The rowing boat rent is 10€/ hour for the first hour and then 2€/hour for the next ones. But if you want to go fishing or catch crabs, you also need a fishing permit from the city. Depending on your age (under 18), which lake you want to fish and with what and for how long, few days permit might be about 10 euros.
I'm sure people who have summer cottages and a rowing boat fish for fun (mostly) in the summer. And even if one doesn't own a boat, there are several places where people can fish on the lake or rapids shore in the middle of the city.