Have you checked out the Navionics webapp for the coastal areas you are referring to?
You can run the Nav+ chart card or in your case, since you own a Navionics card, you would buy the updates card and activate it into a Nav+ card that will allow you to create your own custom mapping cards and be able to use the same card for when you are fishing lakes away from the coast. You simply draw boxes around the areas you would like to fish, download them and go fishing. Once you are done, if you want to go to new areas, you can purge the card and download any other areas you would like. You basically are getting the same inf oshown available on the webapp above. Any other questions just let me know.