Agree. Navionics is the best app available. That is pretty much all I use. If you goto general settings on your iPhone and put auto lock on "never" it works as good as any GPS. However this does drain your battery somewhat, so once you hit your mark, just press your master button and let it run in the background. The app holds around 200 GPS marks and will also track you trip, give you distances, allow you to plan routes, etc. On an iPad is even better than most fixed GPS's.
Good luck.
Cheers scratchie!!!