Re: Hull Water Drain Plug-Screw in Type
Like SBN said... any hardware store will stock either 1/2" or 3/4 " brass plugs. It's probably 3/4". You won't want to put a galvanized plug in a brass "hole" (long term) because unlike metals create an electronic corrosion.
Marinas and some hardware stores will cary Balun plugs - these are the rubber stoppers that fit in and clamp down. In a pinch, ask for "test plugs". These are used to plug plumbing to pressure test before charging water/gas lines.