What on a boat or internal to the boat could result in a reverse polarity light if the dock pedestal, cords and inlets and breakers on panel are all good?
If you have confirmed polarity to the connection point of your boat the next steps is to confirm at the main double pole breaker the polarity that is present. There are a number of causes that can trigger the reverse polarity indicator to illuminate, this is a serious issue and should be investigated as soon as possible by a qualified electrician.