A downcomer for conveying wastewater from an upper level of a marine vessel to a lower level of the marine vessel includes a pair of spaced apart mounting portions and an elongate impact portion extending between the mounting portions. Each mounting portion is adapted for attachment to the marine vessel, and the shape of the elongate impact portion defines a concave space through which the wastewater is conveyed. In one preferred embodiment, a synthetic elastomeric tube, hose, or liner is placed within the concave space, and the wastewater is conveyed from the upper level of the marine vessel to the waterline through the tube, hose, or liner. In another preferred embodiment, the mounting portions and the elongate impact portion are extruded to form a unitary structure including a lightweight, rust-resistant ultrahigh molecular weight polyethylene.