Now that you confirm there's no 240v involved, it is dollars to donuts just some corrosion in the windlass feed wires inside its housing. And you're right, the windlass, rails and rigging are not in the anti-galvanic ("grounding") system. The 24v short is taking a different path to the rails, like down your chain or whatever.
Regardless, this is so likely such a basic and simple fix that you really should do it yourself, if for no other reason than to gain some self-sufficiency for your trans Atlantic crossing ;-) Don't even bother checking with your multimeter - just open the windlass cover and clean up the connections! If you've still got the problem, then spend the money for the electrician, or, better yet, keep looking yourself. Plus, as Ken points out, it's good to do this as preventive maintenance.
Good luck with it,
Craig, SN68 Sangaris, Tropic Isle Harbor, FL