Hi, Amanda ~
Congratulations on your impending nuptials!
Maintaining normal balance aboard a seagoing vessel can be problematic for some AN patients (pre and post surgery) - but not always. I might suggest that your finance try to take a very short boat trip, e.g. a ferry, to get some idea of how his balance is affected on a seagoing vessel. Keep in mind that cruise ships today are really huge, 'floating hotels' and he would have little sense of being on the water, although a storm could occur which might be a problem in terms of his balance. My guess is that the cruise wouldn't be a problem unless your finance has rather severe balance issues, now. As mentioned, a brief foray on a relatively small boat might answer the question because if he doesn't have a problem with balance (or the problem is manageable) than being aboard a large cruise ship wouldn't be an issue for him in terms of maintaining his equilibrium.
Jim