I used my iPhone on the wi-fi on a Royal Caribbean ship sailing the Greek Isles last August. Even though it DOES work on the ship you will get the ship connect charges of about 50 cents a minute. I am not sure what they charge right now.
However, I did not end up spending that much compared to the other poor suckers on the ship. It was torture to wait at satelite speeds for everyone on the ship competing for that tiny bandwith when trying to use their computers or even a laptop to get an email. The internet explorer email interfaces took forever to load each message.
The iPhone just did a quick grab of all of my emails in text format and I was able to quickly deal with them. I never spent more than $1 or $2 dealing with all of mail each day and I was in and out of the computer lounge before some managed to open their first message.
Have fun and do not spend ALL of your time using the ship computers. I was on a 3 week Europe trip and did not want stuff to back up.
PS: If you guys want info on your cruises go check out
http://www.cruisecritic.com you can find out ANYTHING there from others who have taken the same cruises or ships.