Cruises

Atli works in port, not out at sea. It runs on the same mobile networks as any phone on land, and those do not reach across open water.

• In port and near the coast: works normally, as long as your plan covers that country
• Out at sea: no signal. Only the ship wifi or the ship's satellite package work there, and both are sold on board

So buy for the ports, not for the trip. Go down your itinerary and list the countries you actually dock in.

• All in one region: one regional plan covers the lot. A Mediterranean route is Europe only if every port is European, so Egypt, Morocco, Tunisia and Israel need their own
• The Caribbean has no regional plan at all. Those islands are country plans, one per destination
• Spread across regions, or only one or two stops matter to you: buy a country plan for those

You are online only while the ship is in port, so a cruise gets through less data than a land trip of the same length.

One thing to watch: at sea the ship runs its own mobile network and charges its own rates. Leave roaming off on your usual SIM so it does not connect to it.