This is the sms i recieved yesterday on the Irish sea crossing from Dublin to Holyhead. I usually get the same message on any short sea crossing.
Welcome. As you’re roaming on MCP (GNW) (Maritime) which is a Maritime provider, it costs £2.40/min to make a call to the UK and £1.80/min to receive one. Texts cost 60p + your home rate and picture messages are 66p. Mobile data is £7.20 per MB for up to 5MB then £36 for every 5MB after. For terms and info see: m.vodafone.co.uk/ga1 or call +441635691700. For emergency services call 112. Enjoy your trip.
I switch the phone off when it arrives to avoid charges.