El Laucho: is the most advisable beach for the turist, with moderated amounts of people, a sure area to swim (and other not so sure if you dare) and good sand is a nice place to relax. If you want to get wet, I advise to enter by the left corner of the beach, because in all other the floor is rocky. Its also the beach where.the nicest girls ("minas" in spanish lingo) go, and if you are assertive enough, sure will find someone who speak (or try to, as me) english. If no one understand you, body language also goes.
The best and nicest beach in Arica is La Lisera, but the problem is the crowd of people who goes there. When is empty (at morning and usually short past lunch) is a nice place; the best sand and a clear lake-like swiming area, anyway if you dont matter multitudes, go there and enyoy at any hour. Ah! and this beach is special to swim at nigth in summer due its warm waters.
If you love the long, long beaches, sure will enjoy Balneario Chinchorro, there are several miles of beach, when you can enter hundred of meters with water not up your shoulders, moderated to strong waves are ideal for surf beginners and windsurfing, which you can rent in the same beach. In Chinchorro you can see at simple look the turn of South America in the frontier between Chile and Perú.