[chumbucket]

The unit comes with worldwide basemap that has very little detail. It'll show a basic, rough outline of the shoreline and very little other pertinent data. You get the unlock code when you purchase the Bluechart CD. Check Gramin's website for detailed information about chart coverage for your area. The Bluechart software, when added to the GPS, overlays a very detailed chart of the area you are in and receiving a satellite signal from. These overlay charts are exact copies of the NOAA paper charts that everyone uses and should continue to use as a backup. Then as you travel to different areas, your boat appears as a triangular icon that moves over the overlay chart. That chart will scroll continuously to the limits of what you have downloaded into the GPS' memory.
Hope that clears it up for you. Ask away if not and I'll try to explain better. Garmin's are very user friendly though and you'll get it right away. ;)