This is what the Navionics Helpdesk told me when I inquired:
The apps themselves are a one-time purchase for the life of the app, which includes the first year's subscription for chart updates and Advanced Features. After the initial year of purchase, users can continue to use the app as is, without the benefit of updates and Advanced Features. You can find a list of all free features in our online Help article at https://navionics.secure.force.com/k...ng-App-Options
If a customer has gone through the migration process from Boating HD to Boating on their iOS device, any purchased coverage areas (and their subscription status) will be migrated into the new app. If the customer encounters an issue with their subscriptions not transferring properly, we suggest they contact Navionics through the app by going to Menu > Submit Feedback.