A waterproof cover for electrical devices such as electrical outlets, receptacles, switches, ground fault circuit interrupters, dimmers, etc. The waterproof cover has snap-in modular cover plates which can be used either alone with one gang covers or outlet boxes, or ganged together in combination for use with two gang electrical covers or outlet boxes. The cover plate has a side edge which defines an overhanging interlock extending along a first length of the side edge and an underhanging interlock extending along a second length of the side edge. The arrangement is such that the overhanging interlock can interlock with an underhanging interlock and the underhanging interlock can interlock with an overhanging interlock of a second similar cover plate to form a two gang cover plate for a two gang weatherproof cover. Alternatively, the overhanging interlock can interlock with an underhanging interlock and the underhanging interlock can interlock with an overhanging interlock of a one gang waterproof cover. At least one other side edge of the cover plate snap fits under at least one retention boss in the cover, and in a disclosed embodiment three other side edges of the cover plate snap fit under retention bosses in the cover and are secured between the retention bosses and a seating flange.