Just to clarify: WiFi is available throughout the ships, and is free (er, included in the cruise fare) to connect. The ship WiFi is what enables use of the Navigator App, the messaging within the app, and the limited set of free, internet-based messaging apps (iMessage, etc.).
However, the internet access available through the ship WiFi is not free.
I don't make this point to be pedantic; I've seen people get confused, and think that use of the app or other WiFi-enabled things incur additional costs.