The area between Lavrion and Sounion is residential but there are several beaches, coves and rocky places to swim and since in Greece you can't legally own the beach you can swim anywhere. It is just a matter of finding where the beaches are. But if you keep your eyes open and are adventerous you will find them. I found this one with the help of Christos, my taxi driver. I f you want to go to a  beach that will make you think you are on an island instead of eating in town go to Asimakis Beach, which has a nice fish taverna right on the beach. It is on the road between Lavrion and Sounion across from the Hotel Saron and there is a big sign for it that is hard to miss.