Can I install Avalon Offshore on several devices ?

Yes, on all devices of the same type (either iOS or Android) and if the Apple or Android identifier is the same as the one that has been used to purchase the app.

This is valid for the app itself and the in-app purchases (Premium Option and nautical maps)

Apple and Google are competitive companies. If you need Avalon Offshore on 2 devices (iOS and Android), it is necessary to buy one iOS version and one Android version.