If you aren’t purchasing internet etc the messaging app is free to use.
1) download app at home on your device in your own name
2) once on board, put your phone in airplane mode, then connect to the ships wifi (this is free)
3) the app does not provide notifications so you have to keep checking the app for any messages (it’s a pain…but it’s free)
4) older phones you may have to keep reconnecting
5) the app is glitchy for all things not just messages so some patience is needed.
6) fingers crossed that they make some improvements to the app.