In Costa Rica, public nudity is not allowed and since all beaches are public, there are - officially - no nude beaches. Depending on where on the coast you will be staying and when, it might very well be possible for you to find a secluded stretch of beach where you can enjoy the sun clothes free. Be sure to have something within reach to cover yourself with, should somebody come walking by.
Have a great vacation in Costa Rica - land of sunshine and smiles.