Black Water (2008)

"What you can't see can hurt you."

Based on true events, Grace, her boyfriend Adam, and her younger sister, Lee, are on holiday in Northern Australia when they decide to take a tour down a river. As they drift into a swamp, their boat suddenly capsizes. Stranded in the flooded swamp, the three tourists must figure out what to do to survive as they realize they are being watched through the black water.