I just tried iSailor. Navigated a narrow 11 mile channel "Swinomish Channel" from Anacortes to Skagit Bay with out incident. The app was free for my iPhone, but the regional map was $15. Did a great job. The app requires you to acknowledge you are not using it for navigation as the app is not using approved mapping software.
It works well as an aid to dead reconing and a real chart. Indicated there was a bridge in the passage but no hieght information. Needed to refer to my chart to confirm that the mast would clear. Kept us in center channel and provided accurate speed over ground. Was easy to set way points and track the route.
Thumbs up for this app.