~ 5/16" with a washer gasketed with butyl tape. Push it in from outside and put a nut on it. If you use the nut to push the plug inside, you'll hardly leak a drop. This would probably be safe for a season. I have seen plugs like this serve in 500,000 chemical storage tanks for years... which is a little frightening (the gasket material varies with the chemical).
I like this idea a lot. I think this, combined with some underwater-curing marine epoxy, and I'll have a way to get through the season without hauling. And then after the fall haul, I'll go the Don Casey fiberglass repair way.
Of course, I've been wrong before. Like yesterday, when I didn't think about the drill actually going beyond the cradle to the hull, and beyond again....
Thanks, everyone. Well, almost everyone!