Boats that are hauled for the season and relaunched, and trailered boats that are hauled and relaunched repeatedly during the boating season can be subject to oxidation of the antifouling coating if the bottom dries out.
If this happens and you launch the boat, the antifouling biocide can not leach properly, and biofouling organisms have an opportunity to attach. No need to repaint, but before relaunching a trailered boat on which the antifouling paint has oxidized, abrade the surface lightly with a Scotch-Brite™ pad to remove the oxidized surface layer. This action exposes a fresh layer of biocide that will be effective once you splash the boat.