Rife with twists and turns as sharp as any riverbend, Into the Water is a powerful evocation of human instincts and the damage they can inflict. But beware the water's calm surface--you never know what lies beneath.      A flowing river ejects the body of a woman onto its banks: a single mother, the motive behind her death unknown. Weeks earlier, the body of a teenage girl washed upon the river's shores--motive for death yet unknown. But the river has dredged up more than jus