Rosario's best beach is Balneario La Florida, with services and a safe bathing area. Picturesque stalls behind it sell river fish. The summer-only 'Linea de la Costa' bus heads here from Rioja/Roca. Otherwise take bus 153 Roja from the corner of Cordoba and Corrientes in the center. Alternatively take 102 Negro, 103 Negro or 143 Negro and walk a few blocks east from Blvd Rondeau.
Outside peaks season, the beach is unattended and you can visit for free.