There was this similar thread recently and I thought the advice someone gave worth thinking about.
How do I find an employee on a cruise ship?
They suggested contacting the maitre d' on the ship (there is a forumula, ship's initials followed by carnival.com or some such) .. someone else is bound to know how to contact him exactly.
Without knowing you or why you want to contact the person, he might not want to give out his last name if asked, some might not and just use their first names for a reason.
Even if you had his last name .. how would you contact him?
But of course someone who will be on that ship go ahead and ask if there is someone going to be on that ship soon. I'm not trying to discourage that method, just pointing you to a similar thread and the advice given. There are other people also giving advice, like contact John Heald's blog and see if he will forward your contact etc in that thread.