I am betting your boat has a MDC-1600 sending unit in it. We have a 2000 supra that had the exact same problem. This unit was put in damn near every manufacturer's boat made around 2000. I know off the top of my head it was in supra, moomba, malibu, and mastercraft. Check and see if you have this piece if you do the cheapest place I found scouring the web and calling dealers is
$242 shipped. Local dealer wanted $450 for it.