Hatchet
(2006)

After Ben (Joel Moore) and Marybeth (Tamara Feldman) kill Crowley, they get on a boat. Marybeth falls into the water and gets her feet tangled in some weeds. She takes the arm reaching down from the surface, and is pulled up by Crowley.

Ben is dead in the boat beside her. As Marybeth screams, the movie ends.

Thanks Jon!