The ship cellular system is usually not part of any roaming included with your plan.
which leads you to
There is a link at the bottom of that page that says Cruise Ships. Click that link and start typing Carnival and finish with your ship. It will tell you the cost/options.
Voice looks to be $2.50 a minute, texts are $0.50 each, and you can buy a data package from Sprint for $60 for 100MB and $100 for 250MB.
It is generally best/cheapest to put your phone in airplane mode and purchase a WiFi package from the ship if you need to remain connected.