I've never been on a cruise, but I can tell you that you can't keep another person from cheating on you no matter what the situation. May not be what you want to hear, but if they don't want to be faithful you can't put a gun to their head and make them love you. Listen to someone who's been there, you'll be much better off in the long run if you find out she's unfaithful now rather than a few years down the road after your married and have children together