The structures include hollow water-tight bodies. One body is adapted to attach to a boat between the bow and stern of the boat. A saddle area is formed in the center of the body and knee braces are formed on the body forward of the saddle area to permit a person to brace against. A seat is formed to the rear of the saddle area to allow a person to sit erect. The body has hatch covers which allow access to the interior thereof for storage or the like. Thus the body acts as seat, storage area and flotation device. A second hollow body is provided in a shape conforming to the shape of the bow or stern of the boat. This body can be attached to the bow or stern of the boat and acts as a storage area, additional flotation device and/or end cap.