Mobile data on a ferry and on the islands: what works where

Leaving the port: still on land networks

Masts on the coast reach some way out to sea, so for the first part of a crossing, and often for most of a short one between close islands, your phone keeps working on the same networks as in town. A travel eSIM keeps working too, as long as the network belongs to a country your plan covers.

The catch on international routes is that the nearest coast is not always the one you left. On a crossing between two countries, or near a border, the phone may jump to a network of the other side. If that country is outside your plan, the eSIM simply has no data there; if it is outside the EU and your usual line has data roaming on, the bill is another matter. Our guide to crossing a border with a country eSIM explains the switching, and the Greek islands guide covers the classic case of the islands close to Turkey.

At sea: the ship's network is not roaming

Once the coast is out of reach, some ships switch on an on-board mobile network linked by satellite. Your phone sees it as one more network and may join it on its own if roaming is on for your usual line. Vodafone Spain describes this maritime coverage as provided by the vessels themselves, cruises, ships and ferries included, during the crossing, and says it is not included in the roaming of its tariffs; when the boat approaches the coast, the country's normal rate applies again.

The EU price caps do not apply on these networks either, as explained in our guide to avoiding roaming charges outside the EU, where EU operators must also send you a free text when you connect to one. A travel eSIM does not cover ship networks, so at sea the ship's Wi-Fi, if it sells one, is the only way online.

Three settings before you board

• Turn off data roaming on your usual line, or make sure the travel eSIM is the line chosen for mobile data.
• Once the coast is behind you, switch on Airplane Mode and turn Wi-Fi back on if you have paid for the ship's internet.
• Download the ferry ticket, the booking for the island and an offline map of the arrival port while you are still on land.

Airplane Mode at sea also saves battery: a phone that keeps searching for a network it cannot find works harder, and a long overnight crossing is exactly that situation. Ferry and boarding documents saved in the phone's wallet or as files open without data; see keeping travel documents offline.

On the island: same country, same rules

Islands are part of the country they belong to, and the phone treats them that way. The Canary Islands sit off the coast of Africa but are part of Spain and of the EU, so EU roaming and a Spain plan both work there, as our Canary Islands guide explains; the same goes for the Balearics, covered in the Mallorca guide. A country plan for Italy covers Sicily and Sardinia, and one for Greece covers the Greek islands.

What changes is the terrain. Towns and ports are usually well covered, but mountain roads, remote coves and the far side of a hill can be weak or empty. Download offline maps before you leave the port, especially if you are hiring a car or scooter for the day.

Island hopping across countries

A trip that mixes islands of different countries, say Corsica and Sardinia, or Italy and the Croatian coast, needs a plan that covers all of them. A single-country eSIM stops at its own coastline. Snoozim's Europe pack covers 33 countries with one eSIM, so the phone moves to a partner network in each covered country when you arrive; check the list against each port before you travel, because some Mediterranean countries are not in it.

Either way, the eSIM only starts counting at its first connection in a covered country, so you can install it at home and it will wait for the first port.