Just got back from a cruise on Carnival, and your Sprint phone will work at sea. I turned mine on one day at sea to charge it up and unexpectedly got a text message. It told me I would be charged $.05 for incoming and $.50 for outgoing text messages. So you will definitely be charged a premium rate for cruise ship roaming - the roaming that is included with Simply Everything is domestic roaming. I would recommend shutting your phone off or putting it in Airplane mode before you leave the port if you are worried about charges.
Take a look at the details here:
To avoid accidental roaming, I set my phone to Sprint Only networks. From a home screen, tap menu, settings, wirless & networks, mobile networks, system select, home only. This selection may not be the best choice for some folks, but it's a good one for me, considering my personal phone usage. This also would let you turn your phone on to test whether there are any Sprint towers nearby.
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