In the latest issue of BoatUS magazine the article on winterizing seacocks states that "the small screw on the base of the seacock should be removed at winter layup (with boat out of water) will ensure no trapped water will freeze and split the housing.
Surely the author did not mean that this is only done with the boat out of water?
I don't think I'd want to open the drain screw when the boat's in the water. Depending on which side of the valve the drain screw is in, and whether the valve is open or closed.
To me the small screw at the base of the seacock would mean the screw that holds the base to the mounting, a distinctly weird thought. I'm sure the reference was for the plug in the side of the barrel. I find that it does not drain all that well, do the surface tension of water so I insert a pipe cleaner to wick water from the barrel. I am in water year around.