I recently acquired a sailboat and noticed a small protrusion about 5x5in, under the starboard side of the hull in front of the keel. I have heard a few things, the one closest is that it is the plug to seal the toilet that is located above in the cabin The original toilet was removed some time in the past and there is now just a porta-potty type setup. Does anyone know what this could be? Any help is appreciated. Thanks