You can ‘Roam like at home’ on a ferry or plane as long as you are directly connected to a land-based mobile network. For example, if you are on a ship that is travelling away from the coast and you are no longer connected to a mobile network in an EU country, the ‘Roam like at home’ rates will not apply.
Some ferries and planes offer a satellite service to provide coverage throughout your journey. Again, the ‘Roam like at home’ rates will not apply in this case. You should make sure you know the rates for these services before you use them.