Refusing vinegar-soaked sponges offered by spectators, saintly Bennett Beach '71 sprinted his way into the hearts of Boston's townspeople yesterday with a sub-three hour performance in the annual Boston Marathon. Competing for the KISKO KIWIS. Beach finished 32 minutes behind winner Olavi Suomalainen of Finland and well ahead of author Erich Segal '58. "He's a brave lad. Ben is," said race coordinator Jock Semple. "We've been needin' more of his kind, instead of all those fatsos from C.C. with the weird hats."