"Any sort of shark, if they wanted to take a bite, could have ripped that paddleboard. But it didn't," the ranger told Boston25News.
The National Seashore ranger staff and the Atlantic White Shark Conservancy are investigating.
The water was reopened to swimmers around noon, but officials with the Cape Cod National Seashore encouraged swimmers to stay close to shore.
The ranger said she thinks it's very likely that it was a great white shark, but that needs confirmation.
"It was probably a young shark," a lifeguard told Boston25News.
Officials say shark sightings seem to increase between August and October in the northern areas of Cape Cod.