Double Dipping

[Charlie]

I am thinking of stealing power for my wash down pump from the windlass. The cables are already in place and there is a block where I could just add a fuse and a #10 wire then be able to run the wash down pump only when the fuse for the windlass was on and could not run them at the same time.
Pros
This will allow me to use a smaller run of wire. I can use #10 instead of #8.
I won't have to snake the wire thru the entire front end of the boat.
Cons
Can't use windlass and water pump at the same time.
have to turn on the breaker for the windlass to use the washdown pump.
In either case it will correct a wiring job that is not up to par.
Thoughts? Comments?