Not quite true actually. There was the incident of a Boeing Stratocruiser Soveriegn of the Skies which was ditched in 1956. All on board survived. However the main problem with ditching is that the engines on virtually all modern day jets are slung beneath the wing and they would drag the aircraft under on landing. The Stratocruiser was a propeller driven aircraft.
But I said airliner basically meaning a large aircraft. Also your engine theory isn't true. The engine mounts are designed to break out at the rear mount and thus sending the engine up and over the wing. This has happened in most ditching/crashes. The problem is normally the wing tip hitting the water first and spinning or flipping the aircraft.