A travel eSIM on a cruise: keep the ship's network off your bill

Beyond the reach of land networks, a cruise ship can still offer mobile service through a network on board that carries calls, texts and data over a satellite link. Cellular at Sea, which runs this kind of service on cruise ships, says its network switches on once the ship is in international waters, and that a phone whose carrier has a roaming agreement with it will connect automatically, with no action from you. As the ship approaches land again, the onboard network is switched off and phones go back to local service.

The cruise line does not bill you for any of this. According to Cellular at Sea, the charges are cruise ship roaming, billed by your own carrier on your next statement, so the cost turns up on a later bill rather than on your cruise account. The European Commission's roaming guidance explains why that bill can be large: services delivered by satellite systems on board ships fall outside the EU's Roam Like at Home price caps, and they are usually expensive.

If your plan is with an EU operator there is a safety net, though not one to rely on. The Commission says you should receive a text when your phone connects to a network of this kind, telling you the prices, and that mobile services should stop automatically once extra charges reach €50 or another preset limit, unless you have asked otherwise.

The simplest control is also the most complete. Airplane mode turns off the phone's mobile connections, and Apple notes that it can help lower roaming charges while you travel; you can still switch Wi-Fi back on while it is enabled. That combination keeps the ship's Wi-Fi available, if you have bought a package, while neither your home line nor the travel eSIM can register on the onboard network. The European Commission names the same two ways to opt out of roaming on a ship: through the phone's settings, or by turning on flight mode.

If you want the phone line itself to work at sea, that means using the ship's network, so ask your carrier for its cruise rates for calls, texts and data before you sail, which is what Cellular at Sea itself advises. Otherwise, turn data roaming off on your home line. EE notes that this avoids data charges; calls are a separate matter, and its advice for a phone that has joined an unfamiliar network is not to make or answer them while it is connected.

If a line does stay switched on, the less the phone does in the background, the less it can spend. Cellular at Sea's own suggestions for avoiding unwanted roaming charges make a sensible checklist.

The onboard network is switched off as the ship nears land, and phones pick up local service in the port area. That is where a travel eSIM that covers the country you are docking in can carry your data. Cellular at Sea also warns that local service in port may be charged by your home carrier at a different rate from its cruise rate, which is one more reason to let the eSIM do the work and leave data roaming off on the home line.

The handover is not always tidy along a coastline. The European Commission notes that while you are directly connected to a land network, whether on a river, a lake or along the coast, you can still roam like at home; EE notes that in coastal areas a phone can also pick up the maritime networks on cruise ships. As the ship arrives and leaves, check the network name before you use data, and consider doing what EE suggests: switch off automatic network selection on the home line and choose your network by name, so it cannot drift onto the ship's.

Start from the itinerary rather than the length of the trip. List each port with the country it is in and check every one against the plan's coverage list, because on a route that calls in several countries, a plan that covers the first port may not cover the third. With the phone offline between ports, sea days need no mobile data at all, so the allowance only has to cover the hours you actually spend ashore.

Port days are short and the all-aboard time is fixed, so decide in advance what the data is for: the route from the pier to whatever you came to see, a way to reach the rest of your group, and a check on the time. Download offline maps for each port at home before you board, screenshot excursion tickets and meeting points, and confirm which clock the all-aboard time refers to before you step off.

Check validity against the calendar as well. If a plan's validity is counted in days, the sea days between ports count too, so compare it with the dates of your first and last port rather than with the number of days you will spend on land.