Coverage on cruise ships, ferries, and other boats

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Cruise ships sell their own satellite Wi-Fi as a separate service. Some also run a mobile network on board at sea, and your phone may connect to it by itself. No KnowRoaming plan covers it, and on a home-carrier SIM it's billed at sea roaming rates, which are steep. Keep your home SIM's data roaming off, and watch your status bar for an unfamiliar "at sea" network name. For a Caribbean cruise, the Caribbean plan covers the islands on its list, so you're connected at every port on it without buying per island.

Sailing trips, yachts, dive boats, and island ferries behave the same way. You have coverage as you leave port, a gap in open water, and coverage again as you near land. A ferry between Saint Martin and Anguilla, or between Guadeloupe's islands, is rarely out of range for long. A day's sailing between the Seychelles' outer islands will be. Island-hopping spots such as Fiji, Vanuatu, and the Philippines fall in between, which is why their pages suggest offline maps.

Download charts, maps, tickets, and anything else you'll need before you cast off. Your plan comes back on its own within range of land. The days at sea still count toward its validity. Your plan starts when you connect in a supported country, not when you buy. Your validity days count from that first connection. Unused data expires at the end of your validity period. It doesn't roll over.