Reverse Polarity Light

That warning light is trying to tell you something is wrong. Usually NOT good.
Hot & Neural shore wires reversed & Neural connection bad, are examples.
Both conditions can damage equipment, the boat and you.
30Amp connects melt quickly on the air conditioning cable when dual 30s are used.
50 Amp connects hold up a little better.
Carefully Look over your cable plug ends for dirty or damaged pins. Try another shore power cable if any pin or socket does not look "like new".
Try another shore connection to help ensure you don't have a bad service outlet.

If you don't understand what I'm typing, Stop, Call a qualified marine electrician quickly.

115/130Vac over 30 amps will ruin your day (& more) if mistreated.

Are you sure it's problem related to the Neural wire?

US electrical color codes are in example here;

Another example of a problem could be; If the green wire had voltage on it and the white neutral was fine, the light would come on also.

The light is connected between the white and green wire before (shore side) the ships transfer or service breakers.
Usually, these wires (white & green) are tied together at the dock (or shore) and have the same potential.

One of the most common mistakes made during installing plugs or sockets is miss-placing the black & white wires. Seems simple and of course a veteran electrician could never make this error,,,,,,, rookies mess it up also..

I think you missed the point of my post- Look how I re typed what you did. There is a letter missing

Yep, there is a neural missing, shorted, pinched & defective.
Between my ears.

Darn spell checker. Goes back to speed reading, the (my) mind assumes to many things.

Obviously a problem between the shore power plugs on the boat and the Shore/Gen transfer switch.

It's always been this way or new symptom?
This could be something simple but as stated above (despite my poor typing skills) it MAY be dangerous.
Time to bring in an electrician.

I agree with rcrapps, but before you make the call, check that your connections are solid. Most cable ends have a little lip that sometimes makes a solid connection difficult. Failing that the most likely culprit is the shorepower inlet at your boat. It might very well get corroded from salt or the prongs damaged when connecting/disconnecting the cord. Not a hard thing to change, although I'd have an electrician do the repair.

Friend of mine had the same issue. After many visits from electrician discovered that the shore power was not putting out full 30 amps. Check you shore power and ensure that it is giving full amperage.

From someone who is more in the know about US Electrics than me:

"The shore power ground (green wire) could be disconnected or the ground on the dockside shore supply could be faulty. The reason it does not show up on Genset is because the genset switch separates the neutral and the shore power ground and connects it to the genset frame ground."

Yours statement makes it sound as there are two shore power lines.
Dual cables plugged in to the ship?

Dual polarity lights?

My guess would be factory, however it raises a new issue. If your shore power cord has the ground connected, that could be the sorce of your problem. I've not dealt with 30 amp with the GI, but more than a few people have replaced the ground prong on the 50 amp cord. I was almost one the first boat I ran with a GI. I asked the yard to install a new prong, and they were willing. I happened to be on board when the mechanic was doing the job that was ordered. Our discussion educated me and promptly took that job off the list.

Any update? New curly hair do?

Good test, You still get the light plugged in on the yards power supply?

Did not get response on; What power are you drawing; 2 x 30 amp, 1 x 50 amp @ 115Vac, 1 x 50 amp @ 115/240Vac?
Or send us a pic of your cable end(s).

Skippy J,
Does he have two polarity lights? Think that was a early question also.