is right. You have an AC line coming into the boat from your dock cord to a switch with a GIF. That switch feeds a wall outlet. You plug either the inverter or the charger into the wall plug. If you plug the charger in it charges the battery directly. If you plug the inverter in you are pulling from the battery and back feeding into the AC system via the wall plug into the main circuit panel heating up the entire boat. If you screwed up and plugged them both in you would be creating a continuous loop between the inverter sending AC to the battery charger while pulling DC from the battery. You also have a GIF on the output AC side of the inverter. All I can say is don't screw up, you need a transfer switch. Assuming you don't screw up and assuming you don't get in contact with the hot male plug, I don't see why it wouldn't work. It has worked in the past so what's different. If you didn't change anything during the past winter then it must be either some corrosion or a bad GIF. I would clean all the connections while I was installing the new transfer switch and kill two birds at one time rather then killing yourself or one of your kids. Good luck.