We have Verizon and went on a cruise around 2 months ago.
We did use their travel plan thing, since they technically don't have a cruise plan available for the Disney cruises. Ours was $10 for 24 hours if used, which would basically give us the same type of plan that we have already no matter where we were. We were on a 4-night so we stopped at Nassau and Castaway Cay and had 1 sea day. Both Castaway Cay and Nassau were the $10 ones, which not everyone in my family used, you only get charged depending on how many people actually activate it, if you're traveling with other people in your party who are also on your phone plan.
My dad did call ahead and see and had them activate it for our phones, so we all got the text message before we left about how it worked or whatever. But it was basically when you turned airplane mode off in port it automatically activated the service and it was good for 24 hours with it reactivating itself after that 24 hour mark for an additional 24 hours. The plan doesn't work when you're out at sea, obviously, as you have no cell reception. We didn't really try to see if it did, though, because we figured that there was no point and we weren't entirely sure about the whole roaming thing. We did leave our phones not on airplane mode as we began to pull out from the docks once and I believe we ultimately lost it but didn't try to turn it on in the middle of the ocean.
As for our day at sea, we bought the Wi-Fi package, well my 2 sisters did, and they let us use theirs just for the purpose of us sending Snapchats and not losing streaks, we're a Snapchat family lol (even my parents have one). They actually bought it the very first day we were on the cruise and just tried to ration their time using it each day.
You do get the free 50MB the first day, as long as you sign up by the designated time written on the Navigator. I think it's midnight of the first night? My sisters just felt that they would need more than that, we were split into 2 staterooms, so they each bought the 1GB Wi-Fi package with my youngest sister ending with less than 100MB left and my other sister ending with probably around 200MB.