I would check the polarity of the AC supply. The black wire is the hot wire and will read 110VAC between it and the white neutral wire. It will also read 110v from ground to black as well. In your case, if the shore power plug is reversed, your AC wiring system on the boat would have hot on white and neutral to black. If you have a meter or test light that measures ac voltage, you can test this easily by connecting the meter to a regular outlet on the boat when shore power is connected. Common 110v outlets have polarized connectors. One side is smaller than the other. You have probably noticed that most ac appliances will only plug into an outlet in one direction. The Hot lead is connected to the smaller opening on an outlet. You should measure the full 110v AC between there and the round ground socket. If you measure 110v across the larger socket opening and ground, it's reversed somewhere in the system. It could be the shore power cable, the outlet where it's plugged in, the wiring on the boat, etc.
Here's a cheap and easy way to monitor this when you connect shore power in unknown marinas. Get two AC rated LED's from West Marine, just a couple of bucks each. A green and a red. Connect the green one from the black lead on the shore power side of the main AC breaker and AC ground. When the shore power cord is plugged in,before you turn on the main breaker, the green led will light up indicating the shore power outlet is wired correctly. Wire the red led from the neutral on the shore power side of the main breaker to ground. If the wires are reversed, and hot is now on neutral, the red LED will light up indicating a fault.
Another thing to look for is if the neutral and ground are connected to each other ON THE BOAT. It should be bonded at the main breaker panel of the marina, your house, etc. Some people attach neutral and ground on the boat as well and that can create ground loops that can put voltage on the ground system. This will often cause GFCI outlets to trip when power is applied. If you have a separate AC generator on the boat, the neutral and ground must be bonded there, but switched out when on shore power.
http://www.westmarine.com/buy/blue-sea-systems--led-indicator-lights--P009_273_003_509
This is a link to the LED's I used. I just drilled a hole next to the breaker and pushed it in. It's designed to slip snugly into a hole drilled into the electrical panel - no glue or screws. While I was at it, I added "on" indicator lamps to all DC breakers too.