On Carnival, at least, when you're on the cruise ship's cell service it reads "Cellular at Sea" in the network area on your phone.

On the Inspiration, from which I just returned yesterday, they claim the service only operates while in international waters. I can testify that the service was not operating, and we were on local cell service, while we were in or near any port.

On the bright side, the expense kept people from using their cell phones aboard. I never once saw anyone using their phones while the Cellular at Sea network was showing on my phone.