 On the commentary track from Wes Craven and Sean S. Cunningham from the MGM  DVD, Wes Craven mentions that in the scene right after theyve killed the  girls, in the pan across the scenic view, the pan was originally longer,  leading to a shot of Mari crawling out of the lake and holding on to a tree,  still alive. This would explain how she is out of the water when her parents  find her body.