I did not find Apple iMessage to work on my recent cruise without the room Wifi. I didn't really need internet with a tiny bit of planning though.

The cruise line had its own app and THAT had a chat feature. We didn't find it very reliable, however. You would eventually get messages, but it wasn't instant.

I downloaded my ebooks in advance and installed a small LLM on my laptop. I could read and code but I didn't get much of the latter done.