BEFORE TAKEOFF, THE NON-INSTRUMENT RATED 79 YEAR OLD PILOT RECEIVED TWO PREFLIGHT WEATHER BRIEFING. HE WAS ADVISED THAT VFR FLIGHT WAS NOT RECOMMENDED. THE PILOT DEPARTED WITH HIS WIFE TO ATTEND A FAMILY GATHERING IN DETROIT, MICHIGAN, WITH A PLANNED STOP IN WESTERLY, RHODE ISLAND, TO PICK UP RADIO EQUIPMENT. SEGMENTS OF THE FLIGHT WERE OVER WATER. DURING THE FLIGHT, THE PILOT WAS IN RADIO CONTACT WITH GROTON-NEW LONDON TOWER. HE INDICATED ON HIS LAST TRANSMISSION THAT HE WAS DESCENDING INTO WESTERLY. THE AIRCRAFT DID NOT REACH THE DESTINATION AIRPORT. A SEARCH WAS INITIATED AFTER DEBRIS WAS FOUND BY A FISHERMAN. THE AIRCRAFT WAS LOCATED 5 DAYS LATER IN ABOUT 50 FEET OF WATER ABOUT 5 MILES SOUTH OF THE AIRPORT. ABOUT 15 MILES SOUTH OF WESTERLY AT BLOCK ISLAND STATE AIRPORT, AN AIRPORT ATTENDANT REPORTED A LOW CEILING AND FOG, WHICH PREVENTED INBOUND AND OUTBOUND FLIGHT TO WESTERLY.