Time Out says
The outdoor baths at Marzili, in the shadow of the grand parliament building, are among the oldest in the country, and free to use. Consequently they're a popular draw with the Bernese in the summer, who like to cool off in the green waters of the River Aare right in the heart of the city. The river is fast-flowing – many people start upstream and allow themselves to be carried down – but there are lots of paddling pools, lane pools and diving pools here, too. Get back into the river after the weir and float on around the city peninsula (or hop into a raft) for a sightseeing tour with a difference.