www.necn.com calls the object a large inflatable raft which seems more believable than a flotation device, which makes it sound like it was a life saving vest which is soft and comfy and weighs maybe two lbs.
NECN says a 12-year old girl was walking her dog with her sister, close to a boat launch on RT 130 in Mashpee, Ma, so maybe it was a large towable float that caught some air while the boat was towed down the road?
Something like that could happen with a kayak falling from a car roof rack, too.