Amsterdamned

Opening from his point of view, a murderer looks around the city of Amsterdam at night through the canals the city is famous for. He sneaks into a Chinese restaurant's backdoor and steals a butcher knife while the cooks aren't looking. The killer finds his first victim in a local prostitute who, after refusing advances from a cab driver, gets thrown out of the cab. A baglady watches from a distance as the killer plunges the knife into the hooker and drags her back into the water. The next morning, a tour boat on the canal collides with the body of the prostitute, who has been hung on one of the small bridges. As the tourists scream, the body drags on the top of the boat until an opening shows her bloodied body and face.
Assigned to the case is detective Eric Visser, a hard-boiled detective who is raising his 13-year-old daughter Anneke after a split from his ex. He spends an hour in a bath, comes home late from work, and tends to drink now and again. Nevertheless, Eric is praised as one of the best detectives on the force. His partner, Vermeer, meets Eric at the site where the body was found with fellow cop Potter attempting to get through to the baglady, who tells Eric that it was a monster who killed the prostitute and that it came out of the water.
That night, two environmentalists are taking water samples in an ongoing investigation against a nearby chemical plant. However, when the killer emerges, one of the men is taken underwater and when his partner attempts to grab the anchor, he is horrified to find the man's head on the anchor. Scared, the surviving environmentalist swims to the nearby shore and calls for help when he sees a truck pass by. As the truck driver comes out from a distance, the killer grabs the environmentalist and drags him back into the water.
When Eric and Vermeer see the bodies of the environmentalists, he is convinced that there is a serial killer. Eric runs into John, an old friend from the police academy. John works for the river police and we learn that Eric's ex-girlfriend used to date John before Eric stole her from him. When John asks about what happened, John and Eric renew their friendship and are now partners on the case to find the killer.
During a search at a local sporting club, Eric meets Laura, a museum guide as well as Martin Ruysdael, a former diver turned psychiatrist and Laura's doctor/friend. Eric begins to have eyes for Laura and she begins to like him too. Meanwhile, that night, a salvationist and a young woman on a gumboat the next day are the next victims of the killer. As Eric begins to get frustrated with nothing turning up, he and John eventually think they find a suspect in a former chemical plant employee known for his violent outbursts. When the suspect is caught, Eric begins to have doubts. When he sees Laura that night, a skipper at the nearby dock is the killer's next victim with the boat sinking.
John decides to go underwater and investigate the next day. At first, he finds the skipper's body but as he approaches out of the skipper's sunken boat, the killer emerges and a tussle leaves John slashed and killed. Eric arrives to hear the bad news about John. When the killer is located at a marina, a speedboat chase ensues between Eric and the killer. Eric eventually tracks the killer to a local sewer only to be shot at close range in the shoulder with a harpoon gun. When the killer attempts a coup de grÃ¢ce, Eric shoots the mask before waking up in the hospital.
When Laura goes to her appointment and finds Martin isn't home, she stays but hears a noise in Martin's underwater basement. She finds the broken mask and thinks Martin is the killer. An attempt to call Eric fails when he is still unconscious. When he awakens, the nurse tells him of Laura and both he and Vermeer head to Martin's house. Martin arrives and waits for Laura only to hear a sound in his basement. Laura confronts him and hits him repeatedly with an oar. However, the killer comes out of the water and grabs Laura. Eric arrives in time and shoots the killer. Martin finally confesses that the killer is a childhood friend of his and fellow diver who was shunned by society after a commercial diving job caused him to be disfigured by uranium hexafluoride poisoning. The killer arrives at his home, where he reveals himself and decides to take his own life before the police arrive.

Question: What does Laura hit Martin with?
Answer: oar