An automatically sealing boat draining device to be mounted in a hole passing through the transom of a boat, includes: a cylindrical housing with a drain hole; a ball valve assembly mounted within and removable from the drain hole; and a plug for sealing the drain hole. The ball valve assembly includes a ball, a sealing ball seat component, and a flow through ball seat component that are held within the drain hole by a retaining ring. A flow of water from the outer end toward the inner end of the drain hole causes the ball to seat in the sealing seat component and seal the drain hole. A flow of water from the inner end toward the outer end of the drain hole causes the ball to seat in the ball flow though seat component and permit the flow of water through the drain hole.