I'm on a 'family and friends' Vodafone deal which is a really fantatic package and when on board the ferry from Holyhead to Dublin last week, I got a text saying 'Welcome to Maritime Communications - Making calls while roaming on this operator costs €5.11 per minute. Receiving a call costs €3.24 per min. Texts cost 29c to send but are free to receive'
I was fairly shocked to be honest - given how good my bundle is otherwise.
I'd be very wary and wait until I had at least docked at one of the various islands before using the phone - therefore incurring 'regular' roaming charges rather than 'maritime' ones.
HTH