Hi @Dansdad93 we hope you had a lovely trip, but sorry to hear you have a large roaming bill.
As @WelshPaul has explained, it does sound like you have perhaps been roaming at sea, which does incur a large charge.
When you’re at sea, we recommend you don’t use your phone, and put it in flight/airplane mode until you get to shore. This is because using your phone while at sea can be very expensive.
Ferry and cruise operators often use their own phone network that’s operated via satellite. Connecting to a satellite network is much more expensive then connecting to a land operated network.
Have you viewed your bill on the iD app? This will provide you with a breakdown of your charges.
Going forwards you can reduce your bill cap to avoid any large bills. You can view how to do this here - How do I set or change my Bill Cap? | iD Mobile Community
I hope this helps.