Your question is "How do you contact a person on a cruise"? This sounds to me as if the person is on a cruise and you are at home. If that's the case you need to know the agency who it was booked with,
the name of the ship and cruise line. Get those 3 items of information and the contact info should be available from either number one or number three about how you contact number two - by phone or e-mail. If that is not the case and YOU will be on board the ship - then I've wasted both our times!
Hi - if its a crew member find a port of call the ship will be docking at and e-mail the cruise line for the details of the ship's agent at that port of call. They may be willing to pass on an e-mail to the crew member. That usually works for me.