Hey Paul - welcome to iPhone world. I've had mine for a year and a half and love it!
You may have seen my post earlier this week
here about the Navionics Mobile app which has the identical data as the Navionics Gold charts. You would have to check the
Navionics Mobile site to see if you've got coverage. I love the app but agree with the others that a dedicated GPS, whether portable or fixed, is the way to go. The app on the iPhone is a great backup and way to research and plan away from a computer or the boat. I recently picked up a Garmin Oregon 400c as a backup to my fixed unit and I think it's terrific. The included Blue Chart g2 data has the same level of depth and POI (Point Of Interest) info as the cartography in the larger plotters and I would not in any way feel stranded if my main display went down and I did not have paper charts for the area I was in (which would never happen anyway). For inland lakes there's the
400i. And there are obviously a host of other options.
But immediately, if you at least want to give something a shot, see if your boating area is one of the Navionics charts being offered for the iPhone for $4.99. You can't really go wrong even if you try it a few times and never use it again.