Yes. One Tide Guide Pro purchase works across compatible iPhone and iPad devices signed in with the Apple Account used for the purchase. You can check tides quickly on iPhone, then use the larger charts, tables, and planning views on iPad without buying Pro again.
If Pro does not appear on a device, make sure it is signed in with the Apple Account that made the purchase. Then open Tide Guide and go to Settings > Support > Restore Purchase. Restoring checks the App Store for an existing purchase and will not charge you again.
Available plans and family access depend on the current App Store offering and may vary by account or storefront.