there is one I like, Playa la ricarda at el prat de Llobregat. take the train direction Sitges get off at the station El Prat, in front of the station, there is a bus terminal, take the bus P4, and get off at the first stop of the beach, walk back by the seafront direction Barcelona (10 minutes walking). the beach is after the sailing complex. nice beach, quiet, clean water. you must take your drinks, and food as there is nothing on the beach except showers. no ambulent vending neither massage and all the bazar of Barcelona's beaches.