[ILLOGICAL GGC]

sounds to me like you got bad washers on the 2 drain plugs and water is entering the bilge and then the pump comes on and pumps till the battery dies. add gardern hose washers or 7/8 orings to give a positive seal on the plug. i would even consider replacing the plastic drain plugs with S S or brass garbard style drains with brass square head plugs,

also since you own a 186 ,the side scuppers can be letting water back flow onto the deck which in turn finds its way to the bilge under the splash well cover and the same pump running /dead battery scenario. this is esp. true should you have a four stroke motor---been there --experienced that ---got me a 210 instead of the tee shirt