This invention is directed to an irregular in shape thru-hull fitting with specifically an internal seacock which is below the normal waterline of a marine vessel and this invention provides an exterior plug to stop leakage and/or allow for maintenance of the interior hull valve, piping, and/or associated tubing without necessitating the need to haul out the vessel. The seacock plug includes a flexible tubing assembly; o-ring flange assembly, flexible beveled gasket, and compression lead screw with T-handle that forms a watertight seal around the irregular interior shape hull fitting.