Want to do this myself. In reading, it sounds as though the map has to be installed on the device, much like installing new maps on a Garmin for your car. The Navionics App typically uses 3G to update the map on a smartphone as you move, and the GPS inside is used to get your “position” on the updating map. It sounds as though Google maps can be “saved” to a device and then the GPS can show you where you are.
Perhaps Navionics, Lakemaster, or other marine map source is working on this. Perhaps not.
Would love to hear of anyone’s experience in using a tablet (without cellular) for outdoor activities. If you have a tablet with 3G/4G built in, then the Navionics app is good to go.