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A little more info would be helpful. Location and installation of pump, float switch, and discharge hose. With a float switch I assume you leave the pump on while at the slip and you not there. If you do not have a backflow valve (a small rubber piece that fits by the discharge hose attachment on the pump) the water in the hose after pumping flows back into the bilge area. Just enough water to keep the pump recycling itself and get nowhere. It will cook the pump. You might be able to "air gap" the hose but the valve is an easier fix. Too light a gauge service wires might also be a culprit.