They drowned while wading in the shallows near Shreveport.  The victims range from 13-to-18-years-old and are members of three families.  Rescuers learned that none of the six could swim, nor could any of the adults with them.  Investigators believe they got into trouble when one stepped off an underwater ledge into a sinkhole around 20 feet deep.  Each teenager, in turn, ventured out into the deeper water, trying to rescue the others.  One 38-year-old relative tearfully tells reporters how helpless she felt as she stood on the bank and watched the teenagers go under, one by one.  The last victim’s body was pulled from the water around 10:30 p.m. Monday.  The city has scheduled a news conference today at 10 a.m. to release more information.