When Marilyn Monroe first met Joe DiMaggio, famed No. 5 of the New York Yankees, she didn't know who he was. As a matter of fact, she didn't even know what he was, for Joe had already retired and Marilyn had never seen a ball game. Nevertheless, they married and lived together for nine months. Now, after another marriage (to Playwright Arthur Miller) and another divorce, Marilyn has been seeing Joe a lot in St. Petersburg, Fla., where he has been acting as batting coach for his old team. There last week MM (above and opposite) saw Joe in his old Yankee uniform for the first time ever. It was, she breathed, "exciting."