In the SD's Garmin root folder the name gmapsupp.img is universally accepted by marine units. You can check it is working from the Home screen, where you can select and deselect features of the Vision g2 chart via Charts>Navigation Charts.
There is no need for an unlock code and in fact it would even be counterproductive if the img is correctly unlocked. You can check it in GMAPTool, or just run [Only registered and activated users can see links. ] on it if you aren't sure.