A SWIMMER savaged by a shark in the waters close to where the film Jaws was set only escaped after PUNCHING  the powerful predator as it chewed on his leg.

William Lytton was in about ten feet of water off Truro, Massachusetts, when he felt an incredible pain shoot through his left leg and realised he was being attacked.

The 61-year-old neurologist said he then gave the animal a "strong smack" in the gills - a move that probably saved his life but also left him with torn tendons.

He now sports an arm cast on his left arm as well as bandages and a brace around most of his left leg following his brush with death on August 15.

"I initially was terrified, but, really, there was no time to think," he said, recounting his ordeal  at Spaulding Rehabilitation Hospital in Boston.

"It doesn't feel like I did anything heroic. A lot of this was luck."

The New Yorker says he recalled from nature documentaries that the predator's gills were one of its most vulnerable parts.

After the animal broke its grip, he took a few strong stokes back to shore where he shouted for help.

Someone alerted his wife, who had been on the beach with their two young daughters and family friends.

Other beachgoers, including off duty nurses and other medical professionals helped stem the bleeding and carried Mr Lytton up the dunes to the beach parking lot as he started to lose consciousness.

"The pain was really excruciating," Mr Lytton said. "I remember the helicopter landing and then nothing for the next two days."

Mr Lytton was airlifted to Tufts Medical Center in Boston where he said he was placed into a two-day coma, underwent six surgeries and had nearly 12 pints of blood pumped into him.

The professor at SUNY Downstate Medical Center in Brooklyn said he's grateful to be alive, but knows he has weeks more of rehab and at least one more surgery before he's back on his feet.

The shark missed critical nerves and veins but pieces of teeth were cleaned out of his wounds and his bandages cover "hundreds" of sutures used to stitch back together muscles and skin.

"It looks very artistic," he said of his battle wounds.

Mr Lytton was transferred on Sunday to Spaulding, a rehabilitation hospital where many of the most severely injured survivors of the 2013 Boston Marathon bombing were treated.

He is expected to be there around two weeks before eventually returning home to New York for possibly more rehab.

Mr Lytton said he isn't in any rush to wade back into the ocean waters off Cape Cod, where he spends nearly every summer doing research.

"It's kind of terrifying thinking about it," he said. "I know it's not the best thing to say, but I didn't like sharks before, and like them even less now."

Mr Lytton's wife, June, said she hopes the attack is a warning for others to take shark safety seriously.

"It's still not real to me," she said Tuesday standing at her husband's side. "It happened so close to shore. I never thought that could happen."

State biologists are working to determine what type of shark was involved in the attack, which was the first in Massachusetts waters since 2012. The state's last fatal attack was in 1936.

It was the first recorded shark attack at the popular summertime tourist destination since 2012.

Gary Sharpless, assistant harbour master, said the attack happened on the Atlantic Ocean side of the peninsula.