Usually the recommendation is to address the envelope to the crew member (and maybe his/her position such as "bartender") and ship, and send it in care of the cruise line, to the cruise line's U.S. address. They say they forward the letters on to the appropriate ships.
Hi, and welcome to Cruise-chat.com!
If you call the ship, I guess they will be able to find the person you wanna talk to, or put you through their cabin phone.