Expert Advice from wedding guru, Sharon Naylor

Answer:

Fabulous question! This is one best solved by your creating a personalized wedding website -- which you can get for free at several bridal and registry sites! On that personalized wedding website's page about the wedding info, that's where you'll have plenty of room to put the ship's name, the address, a link to the cruise line's site for all the info your guests need about choosing their rooms, passports, etc.
Then, on your invitation, it would read:
...
[date]
aboard the [Ship's name]
with reception to follow

And then you can include a pretty card insert reminding guests of your personal wedding website URL, such as a matching card saying:
"For more information, we invite you to visit our personal wedding website at [URL]" and that's where guests will find out the location and their travel plans, as well as your registry info and links, a little more about your history as a couple, even the dress code for the reception, since many couples know their guests want to know!