A bear swam towards this boat, jumped aboard and attacked a man at Port Renfrew Marina on Vancouver Island on Tuesday evening. (CBC)

A bear mauling a man on a boat in Port Renfrew, B.C., wouldn't let go of its victim despite being struck with a gaff, beaten with a hammer and stabbed with sharp objects, one of the rescuers said Wednesday.

"It didn't let go until it died right in his boat," said Bruce Miller, who was the first to try to stop the attack.

Two men were fishing around 5 p.m. Tuesday when a black bear swam towards their boat and jumped aboard, RCMP Sgt. Roger Plamondon said Wednesday.

One of the men threw fish overboard, hoping to distract the bear, but he was still attacked, Plamondon said.

Bruce Miller says he struck the black bear 10 times with a gaff but it wouldn't let go of its victim. (CBC)

An unidentified 52-year-old man from Saltspring Island was taken to hospital with bites to his shoulder and arm, police said. He underwent surgery and is expected to fully recover.

Miller said he was working on his boat at the Port Renfrew Marina when he heard a "commotion."

"I just happened to look up and this bear was coming out of the water onto the dock, and right into the back of the guy's boat and went right at him," he said Wednesday.

"I had to take a second look … and the guy's friend or his brother was standing on the dock screaming."

Miller ran over with his gaff, a long pole with a sharp hook, and tried to fend off the attack of the bear, weighing about 136 kilograms.

"I gaffed him probably 10 times and was pulling on him and he wouldn't let go of the victim," he said.

"Two or three other guys came over to help and put more sharp stuff in it and gaffed it and beat it with a hammer and it wouldn't let go. Finally another gentleman came over with his filletting knife and cut the bear's throat."

Police said a conservation officer went to the scene and was investigating what prompted the attack.