It was a result that shook the tennis world. In the late summer of 1986, in the U.S. Open’s first round, Paul Annacone of East Hampton defeated John McEnroe, handing the champion of seven Grand Slam singles tournaments a shocking loss.

Early in the morning of Aug. 7, four swimmers — two with considerable open water experience and two with much less — met at the Ship Ashore Marina in Sag Harbor, shook hands, and, in the company of two support boats, set off at 5 a.m. from Cedar Point toward Gardiner’s Island, eight and a half miles away.