User blog comment:Kate.moon/Chamber of Secrets Clip on Deathly Hallows/@comment-97.103.234.106-20110703234702/@comment-173.181.21.105-20110704025310

the water is what Voldemort does so that Ron and Hermione can't destroy the horcrux. And when it moves away from them is when the horcrux is defending itself. Like the diary.