Abstract: A handheld tool is for facilitating installation of a resilient, elongate, generally rectangle-profiled gasket designed to tightly fit inside and along the partially obstructed perimetric channel of a watertight doorway onboard a marine vessel. As typically embodied, the inventive tool includes a straight handle (e.g., shaft or rod) and a head. The head includes a right-triangle profiled section and an L-shape-profiled section. The handle is perpendicularly connected to the hypotenuse surface of the right-triangle profiled section. The L-shape-profiled section has two interior surfaces forming an interior right angle that faces away from the hypotenuse surface. The bisector of the interior right angle is parallel to the handle and perpendicular to the hypotenuse surface. A user holds the inventive tool by the handle, controls the gasket so that a rectangular corner portion of the gasket fits inside the L-shape-profiled section's interior right angle, and forcefully manipulates the gasket into the channel.