Doc B.. I think that is a great idea. Water pumps and impellers fail just like fuel filters get clogged. As others have said you need good maintenance and you canít plan for every unforeseen problem. With that said engines stop unexpectedly and I would venture to say that most of the time itís from fuel filters or the cooling system. Itís happen to me and to friends, i'm sure others as well. I have a dual Racor setup but never thought about this even though i carry a spare impellar and water pump.
A decent sized 12V self priming water pump would be the way to go. As you described itís a very easy setup to plumb in. If you have an additional thru hull that is convenient plumb it there. This is something that should be kept very simple as it would only be used in an emergency situation.
Thanks for passing on the idea.