Hi Eric,
This discussion reminds me a bit like picking flyshit out of the pepper. No big deal either way, nor any "right" way, nor any need for much disagreement anywhere. And I'm not keen on debating how many angels dance on the head of THIS particular pin!
My premise in my posting was mostly based on many, many years experience with both above-water, and underwater screwed fastenings into a variety of woods in a variety of applications. In particular, I seriously doubt that there are any carvel planked wooden boats that are fastened using a sealant of any kind. And those underwater fasteners are submerged 100% of the time. Yeah, the wood gains moisture, both through the fastener holes, and through the wood grain itself. This moisture tends to swell the planking, rendering the fastener watertight.
Same principal applies in Tom's original posting-the rub rail will absorb moisture, both through the fastener hole, and through the rail itself. So what? The moisture absorbed via the fastener will NOT promote rot, nor degradation of the rail, to any perceptible (or measurable) degree. It's an inevitable fact of using wood near water. Using a bit of sealer on the screw before installation will do little, if anything, to change the physics in this application. It will simply add some hassle factor in application of, and cleaning of, the sealer.
I think there's a common misconception that wood must remain 100% watertight, firmly sealed to penetration by moisture, to have a long life. Tain't necessarily so. But again, in this application, no big deal either way.
Very respectfully yours,
Pete