How messages reach someone at sea
You write from shore over the internet; the last leg to their phone happens over the mesh aboard the ship.
You are on land with normal internet. They are on a ship, possibly in a stairwell, possibly in a port with no roaming.
Your message travels the ordinary way as far as the ship, and then the last leg happens over the mesh — phone to phone — until it reaches them. That last leg is the part that works where a signal does not.
Two things follow from that, and both are worth knowing before you worry.
Delivery is not instant in the way a text message at home is instant. If they are somewhere quiet, the message waits for the ship to move around them.
And if you write from the web portal rather than the app, the message is not sealed between phones the way an app message is — see the article on who can read your messages. For most family messages that does not matter. For some it will.
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