The incident occurred around noon ET at Newcomb Hollow Beach, Wellfleet police said.
The Cape Cod beach was closed to swimming Saturday after the awful incident.
"I saw everyone working to get him out of the water and up the beach", said eyewitness Andrew Jacob, 39.
"I was that guy on the beach screaming, 'Shark, shark!'" said Mr Booth. It was like right out of that movie Jaws. "This has turned into Amity Island real quick out here".
If confirmed as a shark attack, it would be the first death caused by a shark in MA since 1936, according to sharkattackdata.com, an online database.More news: Vehicle driver plows into crowds in southeastern France, 2 wounded
The Wellfleet Police Department, which is working with the National Park Service and the State Police on the case, said it is withholding the man's name until the family is notified. The local district attorney's office and the Massachusetts State Police were investigating.
Newcomb Hollow Beach, where the victim was swimming, has a sign saying "Be Shark Smart" warning swimmers to avoid going near seals and to swim near the shore.
'The inshore waters off Wellfleet are a feeding ground for Great White Sharks.
Booth said others on the beach attempted to make a tourniquet while others called 911.
In August another man, 61-year-old neurologist William Lyton, was attacked by a shark just six miles away from Saturday's incident.More news: Google defends Trump election reaction
He sustained severe injuries to his legs and is still recovering in hospital.
The man suffered "injuries to his lower extremities", an off-duty lifeguard told The Cape Cod Times.
Mr Booth said the attack victim and his friend were boogie-boarding when the attack happened.
Barnstable County Commissioner Ron Beaty said he anxious this would happen.More news: Liverpool hold off Spurs to maintain ideal start
- Washoe County Human West Nile Case Most Likely Acquired Locally
- National Hurricane Center Updates Current Status of Tropical Storm Florence
- Rugby Championship: South Africa stun New Zealand in Wellington
- Impressive Chelsea, Liverpool maintain lead over Man City
- Jamie Dimon doesn't have the "smarts" to be president
- Florence 'unloading epic amounts of rainfall' in North Carolina
- Browns' Gordon (hamstring) out vs. Saints
- Prince Harry's wedding ring is different from other royal men
- Texas board of ed drops Hillary Clinton, Helen Keller from history classes
- Two Koreas open joint liaison office in North - pool report