All you can do is say "I'm going on this cruise. If I have OCD while there, so be it. I'll handle that then. And even if OCD rears it's head, it won't be the end of the world"
ocd has been prensent with me with many things that were supposed to be fun...sometimes it dulls the experience and sometimes it's just background noise. But, there are also many times that I think it's gonna be an issue and ruin the experience, but I actually get so caught up in the moment that it hardly affects me and I actually have an amazing time!
You're going on that cruise and it's gonna be an experience no matter what happens!