The UK portion of the account, which is handled by a smaller agency called Positive Thinking, is unaffected by the appointment.
Royal Caribbean is looking to expand its UK advertising and build a global brand.
The agencies will develop work for two areas: the home ports, where the ships are docked; and the fly-cruise market, countries to which customers have to travel to join cruises.
In November, the company announced record net income for the third quarter 2007 of £190m, compared with £164m in 2006, on the back of better-than-expected late bookings.
Launched in 1968 by a conglomerate of three Norwegian shipping companies, Royal Caribbean travels to destinations in Europe, the Bahamas, Asia, Canada and the Caribbean.