I've been using Navionics the past 2 seasons in NY when diving off my boat. Works fine, generally more accurate and informative for finding wrecks than entering GPS headings into the Garmin from fishing,diving and public GPS data websites; however when I took the boat up to the Adirondacks and boated in a few lakes last week it was unable to plot courses because it "thought" the water wasn't deep enough. Still quite useful for finding my way around the lakes though, even without a plotted course. Much better than the Garmin chartplotter.
When I rotate the iphone so the screen is landscape the Navionics app does not display it's dashboard showing speed, so I need to use a split screen on my Garmin rather than using the entire screen to view the sonar.
I put the 2 issues into tech support for Navionics and received a confirmation email "we will get back to you shortly" but they haven't responded- it's been just under a week and I'm a paying customer- it's a yearly subscription as of this year, before that it was a $10 one time purchase/download.