the issue with navionics and sak or any lake that the level varies is that you cannot adjust your offset to compensate for it AND sak was mapped at a very low level and did not get any of the elevation data for what was then an island and is now currently just a couple feet under water and 2 years ago 13 feet under water... even after all the issues I've had with humminbird customer service, their mapping and map chips are the best I've seen... I don't know what garmin has, being they got most of their reputation based on gps and mapping they should have some killer maps... I would assume that since you are on DL you would want something like the humminbird maps because they took into account land above the high water marks and mapped it too which allows you to adjust up or down depending on the depth you see that day... navioncis does give you a good idea of what is down there (better than flying blind).
I guess what I am getting at is, you get an OK map from navionics but if you want the detail and know "hey if I move 15' feet in said direction, I will run into a rock pile or a point" then you are going to really want better maps and you want them to have 1' contours, hell if I could get 6" contours thats what I would be after