A releasably retainable fresh water flushing adaptor and method of manufacture thereof for outboard and stern-drive marine engines having an external lower propulsion housing which includes at least one fresh water intake port along the bottom forwardly bulbous portion of the lower housing. The flushing adaptor includes a molded cover which is mateably, releasably self-retaining over this forwardly portion of the lower housing and also includes a fitting connected or connectable into the bottom forwardly portion of the cover for releasable attachment to a supply of fresh water. The fitting has an internal passage in alignment and in fluid communication with the housing water intake. A water distribution groove formed into the interior surface of the cover adjacent the fitting for an increased water flow into the water intake is also provided. Releasable strap means either connected to or completely circumscribing lower housing and cover may also be provided to eliminate any tendency for the cover to be forced from disengagement with the lower housing by engine vibration or water pressure.