Re: swimming 300 yards off shore IN THE OCEAN.
That was my first take, but the article actually says “300 yards south” of a specific beach.
It sounds like he may have been swimming in an unguarded area, possibly alone, which is also a dangerous invitation to become lunch for some sea critter.
I don’t know what you mean by guarded, but after Labor Day, the beach life guards are no longer employed (paid by the towns).
https://boston.cbslocal.com/2018/09/15/cape-cod-shark-attack-newcomb-hollow-beach-wellfleet/
After a series of shark sightings and the attack in Truro, the Orleans harbormaster warned swimmers they needed to change their habits Experts said at the time aggressive feeding close to shore was expected to continue into the fall months.
In 2017, a shark bit a man’s paddle board off Marconi Beach in Wellfleet.