A convertible cover for protecting a personal watercraft and increasing the carrying capacity of the personal watercraft. The convertible cover consists primarily of a watertight, unitary hull which, when placed on the personal watercraft, encloses substantially all of the upper portion of the personal watercraft to protect the part of the personal watercraft above the hull and the billage seam. To increase the carrying capacity of the personal watercraft, the convertible cover can be placed in the water and towed behind the personal watercraft. Cargo or a seat can be placed within the hull.