One of the best places to swim in all of Attika are the coves at Varkiza. Great snorkling and if you going during the week they are not too crowded.
During weekends and in the summer they are packed. But you can usually find a rock to call your own. You can get here from Akadamias on the E22 or the 115,149 from Glyfada. From the Dafni metro station you can take the 171 which goes all the way to Varkiza.