Does iMessage work on a cruise ship?

Only with an internet connection, which at sea means a wifi package somebody has paid for. The bigger issue is what your iPhone does when iMessage cannot get through: it may fall back to ordinary SMS over the ship's mobile service, and that is billed as maritime roaming.

Blue bubbles, green bubbles, and the bill

A blue bubble is iMessage travelling over the internet. Without a connection it simply does not send — it sits there and goes when you next connect.

A green bubble is SMS, and that goes over the mobile network. At sea the only mobile network is the ship's own satellite-backed service. It is roaming, it is maritime roaming specifically, and it is not covered by your normal allowance or by the EU-style roaming rules you may be used to in Europe. Rates vary but they are high, and people have come home to three-figure bills from doing nothing more than replying to a few texts.

The setting that prevents all of it

Airplane mode, on, as you board. Then turn wifi back on by itself if you want to use a package. That combination keeps the phone off the ship's mast completely while still letting it use wifi.

Two more worth doing while you are in Settings:

• Turn off "Send as SMS" (Settings → Apps → Messages). That stops the fallback happening at all, so a message either goes as iMessage or waits.
• Turn off mobile data as a second layer, in case airplane mode gets switched off by accident.

Reaching people on the ship

If what you actually want is to tell your family you have got a table, none of the above applies. CruiseLink passes messages straight between the phones in your group over Bluetooth and the ship's local network. There is no internet involved, so there is nothing to pay and nothing to bill.

It reaches only the group you set up, everyone needs it before they sail, and a message travels when phones come near enough rather than instantly across the ship. The full picture of what texting costs at sea is here.

Get CruiseLink

Free, no account, nothing to set up beyond your group. On iPhone and Android, and the two work together.