I have always gotten motion sickness easy but have never been sick on a cruise ship, however I have found that if you find yourself staring at something that may indicate motion you may have a better chance of getting motion sickness. An example might be watching the curtains move to the sway of the ship or watching the waves as they go by.
I find I never feel motion sickness if I just go about doing whatever I was doing and not focusing on any particular object. If he starts to feel a little uncomfortable just turn and look at or do something else.
Tell your fiance his eyes are to stay on you LOL.