When you're travelling, either by boat in international waters or in an airplane, you can assume your device will no longer connect to a ground antenna. Instead, it will connect to a satellite, which will significantly increase cost. So note: it's fair to assume your device will connect to a satellite as soon as your feet leave the ground.
Want to have an idea about those costs? Then head over to the calculator on our roaming page and select "From: Satellite connection" (you'll find that option among all the countries) and the country you would be calling.