I've fallen in the waters of the Delta while stand up canoeing and honestly it was refreshing and quiet nice in the summer. The water they drink or cook with in the Bayou Portenio of the Delta del Tigre is sometimes filtered using the same mud that turn those canals waters brown. After all it's upstream the city and irrigated by the huge torrent of the Parana with mostly untouched natural marshes immediately upstream (Lechiguanas Is).
Better river waters are found on the Left Bank of River, Colonia Department of Uruguay, there's a nice beach in Carmelo and another immediately east of Colonia (way to Riachuelo, Co)
If you want better, almost maritime beaches for swimming, Carrasco, a suburb east of Montevideo is quiet clean and as much sea as river estuary, you can see the color of the water changing with the tides, it's perfect for swimming, even light body surfing. It's located in an upscale neighborhood but only 15-20 mins city bus from cheaper overnight alternatives downtown.
Further travel while avoiding high prices or long distances would be either the entire Golden Coast of Uruguay: (Atlantida, Neptunia, Piriapolis)
or "Partido de la Costa" (Mar del Tuyu to Nueva Atlantis) which are the least expensive Argentine ocean beach towns closest to Buenos Aires.