I would go out and buy a second plug in for the GPS unit. If not you might be able to unscrew the plug to get to the wires or you can cut it if you want. Then you should have a black wire and a red wire. the black wire will go to the grounding block and the red wire will go to the switch.

You could also but the cigarette lighter socket at radio shack. Then wire the black end to the grounding block and the red wire to the ACC switch. This way if you want to use the GPS in something other then the boat you just unplug and go. You also get away from having to cut open wires.