You can buy a block of minutes from Verzion, and I believe most major companies. The problems is why buy 100 minutes? I only check messages once or twice, I will pay the rate of 60 cents a minute. I do not think any carrier will sell the roam type minutes that are good on a ship. Those will cost closer to $1.50 - $2.00 a minute, I only check calls when in port.
We did see lost of people using cell phones on ship in the Inside Passage. Unless you are in port those minutes are billed at a very high rate.