BOAT WITH OPERATOR & WIFE ABOARD WAS MOVING SLOWLY ALONG NORTH SHORE OF LAKE WINNEBAGO ABOUT 50 YARDS OFF SHORE FROM THE NORTH SHORE COUNTRY CLUB. WATER IS VERY SHALLOW IN THIS AREA. THE BOAT OPERATOR HEARD A LOUD NOISE LIKE THE PROPELLER STRUCK A HARD SUBMERGED OBJECT. OPERATOR TURNED THE MOTOR OFF AND RAISED (TRIMMED) THE MOTOR UP AND DISCOVERED LARGE AMOUNT OF DAMAGE TO THE PROPELLER AND LOWER UNIT. BOAT OPERATOR THAN 'IDLED' BACK TO THE HIGH CLIFF PARK BOAT LANDING.