Peter, the way I see it is this. Navionics is good stuff but it's basically offered in two main categories, marine charts and lake maps. At the begining there were just marine charts. At that time, not every Tom, Dick and Harry had a chartplotter. Consequently, they were offering the marine charts to people using "navigable waters" which for North America included the entire west coast (Pacific) and the entire East coast (Atlantic). This also included main inland waterways such as the St Lawrence to the Great Lakes. In our case, there were three usable chips for Ontario and Quebec. The 900, 901 and 902 chips which covered some of the great Lakes and the Larry all the way out to the Maritimes. The middle chip, the 901, has Lake Ontario the Rideau System (navigable) the Trent System (navigable) which includes the Kawarthas. Georgian Bay and Lake simcoe was/is on that one as well. Well, wouldn't you know it, the powers that be saw fit to remove the errors in satelite transmission making reception of more accurate location available to everyone. Plotters came down in price and we now had folks screaming to have their favorite lakes available....not an easy task.
So, Navionics adds another series of maps to their lineup and call them lake maps. These are more or less an addendum chip to the marine charts because of customer demand. These lake chips were available for 5 or 6 sections of the States and one section for Canada. The Canadian lakes chip had (very approximate numbers) 40 lakes in BC, a few in Alberta, 4 in Saskatchewan, 80-90 in Ontario and about 120 in Quebec. The maps of the main navigable waterways (marine charts) were the same maps you were buying for $20 or so for a very small section from the Canadian government.
Today, they are adding maps as demand warrants and as they are available to them from what I can see. So, today, if you buy a lakes map, there are chances that there won't be any waters that are contained in the marine charts.
That should be fairly clear......it's only 6:30PM :-)