What movie did Robert De niro play in that he kills a girls parents on boat?

The movie you're thinking of is Cape Fear (1991). In this remake of the 1962 film, Robert De Niro plays Max Cady, a convicted rapist who terrorizes a lawyer (Nick Nolte) and his family after being released from prison.

While Max Cady doesn't actually kill the girl's parents on a boat in the movie, he does threaten them, putting their lives in danger on a boat, and the scene is a very intense and memorable moment in the film.