Has anyone else had this or does anyone know why this would be the case?
Best answer by Mohammed
Sounds like you will have connected to a Ferry Network during your travel.
This network isn't included in the EU Roaming which is why you were charged out of plan usage.
There would usually be signs on the Ferry advising of the charges for accessing their network.
It's best to keep Roaming completely off until you're in the inclusive destination for Roaming to avoid any unwanted charges.