Having sailed several times on 3 of the 5 ships, my experience with chatting via the navigator app has been mediocre. Messages would either be delayed or not come through. The notifications from the app very often would not show up, even when they were turned on in settings.
iMessage worked well, until it didn't. Every so often a phone would de-register from iMessage and would stop working. If the phone's user slipped up and tried to send a picture or to open a picture someone had sent them, it would break their iMessage connection until they logged on a full wifi or cell connection or it spontaneously re-enabled itself sometime later. iMessage would also spontaneously de-register itself at times too such that it would just stop working.
I'm not sure if this is still a thing or not, but we would save the free 100MB that you get on day 1 just for re-registering iMessage.
WhatsApp seemed pretty reliable by comparison, though. It wound up being our go-to for coordinating, even though all of us have Apple devices. Messages and notifications were both prompt and present.
Overall, I'd say WhatsApp worked 90+% of the time, iMessage about 75%, and Navigator about 30%.