Well, this is all interesting. I wanted to download and compare this new US Marine and Lakes--mostly for the inland lakes which I did not have. I did have all of the US and most of Canada, plus caribbean all on the I pad. So....to download the US marine and lakes, I had to upgrade to most recent OS for i pad. Somewhere along the way, I lost nearly all of my apps--and since the apps I had from Navionics were not on the app store, I could not reload them. My fault, but they seemed to be backed up. Not so.
The difference is that all of the HD charts of the US Caribbean, Mexico etc were on my I pad, instantly available, and the shoreline was shown initially on Google earth. I have downloaded from I cloud, most of the areas I am interested in. There are "handles" on the down load box, and you can move the area of down load, to near to what you want. Just after that down load, you can move the curser along into adjacent non downloaded areas, and it will down load. It looks as if you zoom down on an area that it will load the detail chart--but apparently from 'the cloud"--not from the computer. As noted above, once down loaded, the charts stay there.
The Lake Powell chart is about equal to my Garmin Lakes chart, and I compared them side by side--both are at full pool for shorelines, so you have to click on contour lines.
I also checked much of the great loop, especially areas I know well, and the charts seem accurate.