From LA Times:
Pop & Hiss conducted a test run of an hour of continuous music streaming on the default sound-quality setting with Pandora on the iPhone. (Flipping on high-quality streaming doesn't make a huge difference in bandwidth consumption.) It weighed in at just under 16 megabytes.
In other words, you'd need to stream about 4 hours on Pandora every day to hit the 2-gig monthly limit. Mileage varies depending on your choice of streaming service. But as Wired points out in its investigation, many services, including Rhapsody, MOG and Spotify in Europe, allow you to save songs for offline listening. Also keep in mind you can switch to Wi-Fi where available to not count against your 3G data total.
Couple of cruises won't put a dent in your plan