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" and the country you would be calling.