I preface my comments with the warning that I have never done this transfer so maybe others will have more reliable information.
The Frankfurt Airport homepage http://www.frankfurt-airport.com/cms...airport_en.htm
offers helpful information under "Getting to/from the Airport".
It is indicated that a taxi costs about 20 Euros to the city center so I would imagine that a trip to the ship should not be much more unless it is docked in Mainz. Other than private transfers that would seem to be the most reasonable for people with luggage and I doubt that you can get a private transfer for anything close to that price. And remember, the price of a taxi is for two while other modes such as bus or train cost per person.
Hope you have a wonderful trip and that others more familiar with Frankfurt will correct any mis-information that I have provided! Clarence