Pools in rivers to promote access and awareness

The Potomac and Anacostia Rivers aren't safe to swim in, and in order to become so, way more people need to be engaged, interested, and concerned about river health, including how their actions help or hinder river health.

In the summer, why don't more cities have in-river pools?

Yes, the water has to be highly filtered to be able to be used, but considering how disconnected we are from the rivers in the DC metropolitan area (note that in Colonial Beach, Virginia, about two hours south of DC, there are beaches on the river and the water is safe for swimming).

While there have been various efforts in NYC over the years, Berlin might be the most prominent example, with a pool in the River Spree. Called Badeschiff, it has been operating since 2004, and was designed by a team of Spanish architects, Fernando Menis F.A. Rufino, J.M. Rodriguez – Pastrana with Gil Wilk.

In the DC area, this could be done in DC, Arlington, and Alexandria.

There is a prototype project for the Hudson River in Beacon, NY  (River Pool at Beacon) although they have had clean water issues ("Bacteria found in River Pool, but public never notified," Poughkeepsie Journal) and there is a proposal for a boat-based pool for NYC that will move and filter and release clean water into the rivers. NYC's Plus Pool project has been written up in the NY Post.

Paris has a swimming pool barge, Piscine Josephine Baker, on the River Seine.   It's been open since 2006.

There was a floating pool in the Bronx, sponsored by the Neptune Foundation, but I don't think it is in operation.  For certain it operated from about 2007 to 2013.