Living in Southern Catalonia I am not aware of any naturist beaches close to Salou, but a few stations on the rail will bring you to L'Hospitalet de l'Infant which boasts our favourite beach, the Playa del Torn.  Whilst the beach is a fair walk from the station I am sure that the taxi drivers would be happy to drop you outside the Templo del Sol naturist camp and a short walk around the outside in the direction of L'Hospitalet will bring you to the steps down to the beach.  I know that there are also beaches north of Tarragona which are used for naked recreation but am afraid I cannot tell you how easy they are to reach as the drive there is too far for us.
I have a Spanish friend who is a member of this site and has posted a map to naturist beaches in Catalonia on the Beaches (non uk) section of the forum.  You will probably be able to find this by searching for his posts under his user name of Leriel.  Hope that this helps.