No you'll just be left hanging over the side while the prop re-designs your feet.
It could work though, if instead of working like a seatbelt it could be attached to the engine stop, then if you did go overboard, the engine, and therefore the prop, would stop almost instantly. And to allow you to move away from the boat it could be fitted with a shear pin to disconnect you from the boat.
Better still avoid getting chucked in in the first place.
Edit. Actually the more I think about the idea of the engine stop, whilst agreeing it's possibly a bit overkill, I've found a couple of other reasons. if you go overboard and the engine is then stopped, your boat isn't going to go very far before coming to a halt, also if you are on the back on your own then the engine stopping will alert anyone else on board that there is a problem.