THE PILOT AND HIS PASSENGER, HIS SON, DEPARTED FROM A CONTROLLED AIRPORT AT 1436 HOURS. HE CLIMBED THE AIRPLANE TO ABOUT 5,200 FEET MEAN SEA LEVEL. RECORDED COMMUNICATIONS BETWEEN THE PILOT AND THE CONTROL TOWER DID NOT REVEAL ANYTHING OUT OF THE ORDINARY. RADAR DATA SHOWED THE PILOT IN A PROGRESSIVE CLIMB TO ABOUT 5,200 FEET. AFTER MAINTAINING THAT ALTITUDE FOR OVER TWO MINUTES, THE AIRPLANE DESCENDED AT AN EXCESSIVE RATE UNTIL IT COLLIDED WITH THE OCEAN ABOUT ONE MILE OFFSHORE. THE ENGINES AND PROPELLERS WERE RECOVERED AND EXAMINED. THE ENGINE EXAMINATION DID NOT REVEAL ANY PRE EXISTING DISCREPANCIES. THE PROPELLERS HAD DEEP LEADING EDGE GOUGES, 'S' TWISTS, AND ONE BROKEN BLADE. NO PRE EXISTING DEFICIENCIES WERE FOUND DURING THE EXAMINATION OF THE AIRPLANE'S LOGBOOKS AND MAINTENANCE RECORDS. INVESTIGATION DID NOT REVEAL ANY REASON FOR THE AIRPLANE'S RAPID DESCENT AND ITS IN FLIGHT COLLISION WITH THE OCEAN.