NavShip for iOS supports offline maps from OpenSeaMap as of version 1.4.2. As soon as the app contains the menu item “Offline Mode”, the function is unlocked.
Version 1.4.3 makes route saving and loading available.
Installing the offlin maps is very easy. All you have to do is select “Download maps” in the menu and click on the desired driving areas. The app automatically downloads and unpacks the files. When switching to offline mode, the available zones are automatically loaded.
It is recommended that you download the overview map “Europe 1-7” in order to find your way around. The numbers behind the area names are describing the zoom levels. 1 corresponds to the complete world overview and 18 e.g. in the latitudes of Central Europe, a 50m large tile.