Cape Fear

Sam Bowden (Nick Nolte) is a lawyer in the quiet town of New Essex, North Carolina. Max Cady (Robert De Niro) is a former client whom Bowden defended 14 years earlier when he was working as a public defender in Atlanta. Cady was tried for the violent rape and battery of a young woman. Bowden, appalled by Cady's crime, buried evidence that might have lightened Cady's sentence or even secured his acquittal. After his release from prison, Cady tracks down Bowden. He studied law in prison and assumed his own defense, unsuccessfully appealing his conviction several times. Cady hints he has learned about Bowden burying the report.
Several incidents involving Cady impact the Bowden family, which consists of Bowden's wife Leigh (Jessica Lange) and their teenage daughter Danielle (Juliette Lewis). The family dog is poisoned. Cady lurks near their property. Bowden attempts to have Cady arrested, but police lieutenant Elgart (Robert Mitchum), says there is no evidence of a crime. At a bar, Cady meets Lori Davis (Illeana Douglas), a colleague of Bowden with whom she might be having a love affair. At her house, Cady cuffs her hands behind her back, breaks her arm and rapes her. Lori refuses to press charges, ashamed of what happened. Bowden hires private investigator Claude Kersek (Joe Don Baker) to follow Cady.
Cady next approaches Danielle at her school by impersonating her new drama teacher and kisses her. Bowden warns him to leave his family and him alone or suffer the consequences. Cady secretly tapes the conversation with a hidden recorder. Kersek persuades Bowden to hire three men to beat Cady and intimidate him, but as Bowden watches from a hiding place, Cady turns the tide on his attackers and viciously beats them. Cady uses the recording of Bowden's threat and an exaggerated display of his own injuries to file for a restraining order against Bowden. Cady's new lawyer, Lee Heller (Gregory Peck), files a complaint with the North Carolina State Bar, vowing to have Bowden disbarred.
Kersek reasons that Cady may try to enter the Bowden house during Bowden's appearance at a bar hearing out of town. They fake Bowden's departure and hide in the house, hoping that Cady will break in so that he can be shot in self-defense. Cady kills Kersek and a housekeeper before escaping. Bowden, Leigh, and Danielle discover the bodies. Horrified, they flee to their houseboat, which is docked upstate along Cape Fear. Cady follows them by tying himself to the chassis of their car. He attacks the family on the boat, beating and tying up Bowden, and prepares to rape Leigh and Danielle while making Bowden watch. Danielle sprays Cady with lighter fluid while he lights a cigar, engulfing him in flames and causing him to jump off the boat to extinguish the fire. However, Cady clings to a rope and pulls himself back on board.
As the boat is rocked by a violent thunderstorm, a badly burned Cady confronts Bowden with a mock trial. The storm knocks him off his feet, allowing Bowden to gain the upper hand once the women make it to shore. Bowden uses Cady's handcuffs to shackle Cady to the boat. When the boat hits a rock and is destroyed, the fight continues on shore. A raging tide carries Cady away, and he drowns. Bowden performs a cathartic washing of blood from his hands before rejoining Leigh and Danielle farther up the riverbank.

Question: What does the boat hit that destroys it?
Answer: Rock