LTEREDl LAGOON---Dark, jagged volcanic reefs nearly enclose this swimming hole near Arecibo. Waves break through a narrow in the rocks, but the lagoon is largely fed by an underwater tunnel. White coral sand lines the blue pool, while palms wave along ridge in the background. Residents of the hot little town of Arecibo and the surrounding area flock to this lagoon on weekends. Puerto Rico Boasts Fine Beaches .w CAR iBBEAN CAROUSEL TEXT and PHOTOS by THOMAS TURNER TAKING A LOOK from high atop the rocks THEN, DIVING . into the ocean below :+ ::w: