This is from the HD version of Navionics on the iPad at a comparable zoom level to Jon's example. The scale shows 0.6 nm but is also smaller on the screen.

Anyway, the danger depth is set to 10 feet, though I usually have it set for 18 feet. You can see the buoy off Orpheus Rock, and the dark blue indicates a depth of 10' or less. That's a sufficient clue for me to stand clear. When fully zoomed in, it never resolves into a rock as such, just a bigger and bigger dark blue blob. Oh wait... at 2.0 mile full screen scale, the "Orpheus Rock" label appears. Rant on that below.

Overall, I have been satisfied with the accuracy of Navionics (not that I would ever stop referring to paper charts, sailing directions, guide books, local knowledge, the depth sounder, wave patterns, water shading, sounds, etc., etc.). I didn't see a lot of errors on the iPhone last year when we crewed on a friend's boat in the Bahamas. iPad Navionics accuracy in the Sea of Cortez area was as good as any official charting there and way better than the latest Cmap cartridge (NT+ Wide?). The accuracy was still not great in some areas. We could have been in trouble there without marking a lot of waypoints from Sean and Heather's cruising guide and heeding their advice.

There is something that thoroughly bugs me about Navionics charting on my iDevices. The labeling of place names, rocks, capes, obstacles, is downright deficient. Cape Canso isn't labeled until you zoom wayyy out. Other labels appear only when you are zoomed wayyyyy in. Jon makes a good point that any label with "Rock" as part of it gets your attention. I'm sure the electronic chart makers struggle all the time with the shortage of pixels. A big paper chart has almost infinite pixels by comparison. Labels on a small screen can be reduced in size only so far before becoming unreadable. Too many big labels and they start running into each other and... become unreadable. It may be time for some new user interface idea.... rocks that swell and pulsate or something! I'd hate that, but maybe some kind of offshore hazard detector. There must be some more ways to lull us into a false sense of security!!!