In a hypothetical situation with loss of engine power, although it might be possible for a falling helicopter to flip over due to wind shear or some other violent turbulence, I don't recall ever hearing of a helicopter flip over like that. Since the engines are quite heavy and autorotation (even upside down) tends to create drag from the rotors, I think the tendency of an upside down falling helicopter would be to flip back over, rightside up again assuming there was enough altitude and time.

But that's different from the hypothetical I mentioned, where the rotor strikes something that physically stops the rotor so that the momentum and torque present in the rotor suddenly needs to find something else to turn, namely the rest of the helicopter.

Once in the water, however, the dynamics totally change. The body of the aircraft then becomes a top heavy floating object and the weight of the engine and main rotors will want to go to the bottom, the same way that a car in the water typically sinks nose first. It's kind of like trying to lie on top of a big beach ball in the water--the tendency is for the heavy object, i.e. you, to go under while the buoyant ball is pushed upwards by the water.

I've always been amazed that people continued to try and develop the helicopter until they got it to work. Unlike the way a fixed wing aircraft works, a helicopter is just brute force exerted against gravity in a battle to stay aloft. Then again, we do have hummingbirds and other hovering birds in Nature, too.