Calling (and receiving calls) from many countries is expensive
Let's start with defining a difference between roaming and international calls.
Roaming is when a mobile phone is not on it's own network and attached to another providers network in order to provide service.
If you're making an international call, you're in your home country but calling someone who's in a different country. So you'll still be using your home country's network.
So if you are using roaming abroad (calling or accepting the call) you might hit with eye-watering bills. Receiving SMS is generally free, but it will usually cost more to send them back.
Data mobile usage on sea, ferries, cruises, airplanes
It turns out that ferries usually have their very own phone network, operated via satellite. So if your phone can't connect to 2, 3 or 4G network it may seek out the satellite connection, which is much more expensive.
For example, if you are relaxing on a boat on the Greek coast there is a possibility you will latch on to a maritime network which normally charges over $2/minute and $5/Mb.
The cost of data usage
The cost of overseas data use can be ridiculous. Some travelers try to protect themselves from unwanted bills by buying an international data roaming pack from their operator before they go. Though other unexpected troubles may occur. Your data may run out when you need it most forcing you to look for non-secure WiFi or pay high overage fees. You can manage data on your mobile device, but isn't it better to make sure you have connectivity regardless of where you are?
Imagine, you can talk, text, and surf - from anywhere / to anywhere - for a fixed monthly fee with no hidden charges.