First off, is it real Mahogany? (Swietenia) There are many reddish woods labelled Mahogany that really aren't. What type of boat is this? Is the rudder easily removable? Any pictures?
Varnish has absolutely no anti-fouling properties, neither does epoxy. So using either as the final coat below the water line is of no value in that regard.
I would take the rudder off at the dock or mooring, and varnish it. (If that is an option.) Or you could scribe the waterline with the board up, and paint the submerged part, to 2" about the water, and paint that part with anti-fouling paint, or paint the whole thing.