As far as I am concerned the NAVIONICS web site is one of the worst out there with one of the best products. I finally figured how to download the U.S. and Canadian base map and then the charts that go with that (base map is free and then you pay for the chart download). Total cost was about $55.00. Very reasonable for what you get! I downloaded them on my 8" Samsung tablet which is GPS "hot". Since it is GPS "hot" I do not need a provider such as Verizon, AT&T, etc. It is fast and very easy to operate. My wife and I put 1291 miles on our 25SC this summer exploring from Seattle to Bella Bella area and back. Before we left I spent many an hour using the Navionics not only to learn how to use it but to explore areas I knew we wanted to visit.
We have the 5212 Garmin and the Samsung was on the dash at all times. Becasue of the Navionics we explored many areas (espeially the many islands below Bella Bella) that I would not have done without it. I could expand and move to areas we were approaching quickly and then expand that area to search for depths and shoals. And it is comforting when you enter this area to know that if the Garmin goes down I still have a chart plotter.
One Thought here. If you are going to buy a tablet just be sure it has build in GPS or you are going to have to connect to a GPS dongle. I was in Costco the other day and Samsung now has a "new" 8" tablet that looks just like mine. However, for some reason they have eliminated the GPS feature. Go figure.