Two issues. First, wifi calling must be set up where you phone carrier can connect. As long as you have done this once on your phone you should be fine. Second, you don't want your cell phone to connect to the ship tower. They turn this on roughly when the pilot leaves the ship after sailing. If you see "Cellular at Sea" you can incur high costs for data. Easiest way to protect yourself is airplane mode then turn wifi back on.
Wifi gets access to local stuff on ship. You need an internet package to connect to anything off ship, including wifi calling and texting.