A cover for a swimming pool includes a flexible sheet of material covering substantially all of an opening into the swimming pool. A split extends within the cover. Zipper elements line edges of the cover along the split to close the split, thereby preventing debris from entering the swimming pool through the split. A locking plate member covers top and bottom sides of the zipper to secure the zipper during the off-season. The locking member includes spaced apart upper and lower plates, forming a slot therebetween, whereby the slot provides clearance for fabric edges of the zipper. A bridge section joins the plates adjacent to a rear side of the locking plate member. A recess is formed in facing surfaces of the lower and upper plates between the bridge section and a front side of the locking plate member, to accommodate the zipper elements therein.