Witnesses reported that when the aircraft was stabilized on final approach, it pitched up slightly and yawed, then rolled left to an inverted attitude and crashed in a canal. Several witnesses stated that the left propeller was windmilling. One stated the left propeller had stopped. Two others said smoke was trailing from the aircraft. One witness stated that after the aircraft passed over his car, a fine mist of oil fell on his windshield. When the aircraft was recovered from the canal, the left propeller was found in the feathered position while the right propeller was near its low pitch stops. The left propeller dome was found depleted of air pressure and no oil was found in the left engine case. Also, an oil film was found on the bottom of the left engine, left flap, and left stabilizer. A further exam revealed that an oil interconnect line, Piper part number 920024-811, had failed inside its fireproof covering. The hose was 8 years old, and its recommended replacement interval was 5 years. It had been installed with a small bend radius. Also, the left engine had evidence of heat and wear due to lack of oil.