A sealing closure assembly for closing off a skimmer opening for a swimming pool includes a closure faceplate and a cover removably connectable to the closure faceplate. The closure faceplate has a sidewall surrounding a central opening, a flange protruding from the sidewall to overlap a peripheral area of the skimmer opening, and a lip protruding from the flange and extending around an inner perimeter of the flange. The cover includes a cover plate with a sealing gasket fitted around an outer perimeter of the closure faceplate, a wedge plate member for engaging an inside sidewall of the sealing gasket, and a closure member for exerting a force between the cover plate and the wedge plate to pull the cover plate and wedge plate together between a locked and unlocked position. In the locked position, an outer sidewall of the sealing gasket engages the flange (1) proximate to the sidewall of the closure faceplate, (2) at the lip of the flange, and (3) by wrapping around a terminal edge of the flange.