Sorry you were disappointed in the entertainment. Actually, Royal Caribbean does hire professional singers and dancers. They're booked through a professional contractor, often the same one that provides entertainment to several ships on different lines. The shows are developed by professional choreographers and show producers. The singers and dancers are trained at a facility in Florida, and then board the ship as a troupe, and stay about six months. When they work for Royal Caribbean, they're called the Royal Caribbean Singers and Dancers. They're often from different parts of the world, and depending on the contractor, are sometimes mostly British.
These singers and dancers do not double as waiters, room stewardesses, or anything else with the possible exception of helping to run the lifeboat drill the first day onboard. My lifeboat drill on Voyager of the Seas many years ago was lead by one of the ice skaters from the ice show.