Troubled Waters (2004)

Original title: Les eaux troubles

Clara takes her daughter and her friends to a weekend visit at her grandfather's village. They all have a great time, but after they return home people start dying. First Clara's grandfather mysteriously drowns, then the children start getting sick and one, her goddaughter, dies. Around same time clients at the spa she teaches aqua-aerobics start getting sick with one dying. She finds some mysterious algea, related to Caulerpa taxifolia, planted near where her grandfather supposedly drowned. Clara suspects the algea and all of these illnesses and deaths are related to some sort of poisonings of fish caught near her Grandfather's village.