DURING DESCENT ON AN IFR FLIGHT, THE PILOT BECAME INCAPACTIATED. THE NON-RATED PASSENGER THEN MADE THE FOLLOWING TRANSMISSION: 'CAN ANYBODY HEAR ME?', FOLLOWED BY '... HE WENT TO SLEEP ON ME'. RADAR DATA INDICATES THAT N97PA THEN FLEW ERRATIC HEADINGS AND ALTITUDES UNTIL FINALLY TURNING TO A SOUTHEASTERLY HEADING, TOWARDS THE OCEAN. ATTEMPTS BY ATC TO DIRECT A TURN BACK TOWARDS LAND WERE TO NO AVAIL. RADAR CONTACT WAS LOST 4 MILES FROM THE COASTLINE OVER THE ATLANTIC OCEAN. THE COAST GUARD RECOVERED AN AIRPLANE SEAT AND TWO SECTIONS OF LANDING GEAR. POSITIVE IDENTIFICATION OF THE PILOT AND PASSENGER WAS NOT POSSIBLE. THE PILOT HAD RECEIVED HIS CLASS II MEDICAL THE DAY OF THE ACCIDENT. A SEARCH OF HIS MEDICAL RECORD REVEALED A SLIGHTLY ABNORMAL ECG IN 1988 (ON APPLICATION FOR CLASS I MEDICAL). ALL SUBSEQUENT APPLICATIONS FOR FAA MEDICAL CERTIFICATES WERE FOR CLASS II CERTIFICATES, WHICH DO NOT REQUIRE ECG TESTING.