jamming/failure of the left magneto impulse coupling, which stopped rotation of the magneto gear, and resulted in subsequent shearing of the accessory intermediate idler gear. Factors relating to the accident were: the lack of suitable terrain for a forced landing, which necessitated ditching of the aircraft; the passenger's lack of awareness concerning access to life vests, due to the pilot's inadequate briefing and the seat covers being installed over pouches that held the life vests; insufficient company standards/procedures regarding access to life vests.