Abstract: A ventilator for a boat mooring cover or the like including a support plate on the underside of the cover and a cover plate on the outer side of the cover. The support plate and cover each include passages therein in fluid communication with each other and with an aperture provided in the cover. An extensible and retractible support is provided for adjustably positioning the ventilator at different levels. The support plate includes an annular groove while the cover plate includes an annular detent or clamping rib which cooperates with the groove in the support plate to pinch and displace a portion of the cover therebetween to attach the ventilator to the cover.