I don't have any advice on the cruise, because I've never been on one. But thanks to Jennifer for the advice, because I'm contemplating one for my honeymoon next year!
Anyway, have you tried taking the fiber supplement before you go to bed? I think Heather mentioned before that you can take it before bed to help for problems first thing in the morning, so I wonder if this may help you. Also, what dose are you up to on your fiber supplements?