I'd pass it through the rail as mentioned above through something as simple as a grommet. I, personally, would not get too bent out of shape about "weatherproofing" the penetration. Ensure, if your decks are cored, that the hole you make in the deck at the base of the rail is sealed from the core. However ... I wouldn't make the (ineffective) mess of installing a grommet with sealant. It's not going to work very well in this location ... simply due to the combined difficulties of the curved sealing surfaces and potential for relative motion of wire in grommet or grommet in rail if you ever bump the cable.
Any amount of water entry through this penetration would collect harmlessly in the rail foot and if it does happen to drip into the boat ... would be so minimal you might as well call it humidity. It's not going to sink your boat if you don't get a cable penetration through a pedestal rail absolutely waterproof.
1981 C&C 32