The passengers can move freely about the cruise ship but must not leave. This will not change until the results are available. If the tests are positive, the ship could be quarantined.
A few days ago, the Emirates tightened its entry requirements for cruise ships. Only vessels that were already in the territorial waters of the Emirates before the changes to the regulations may enter.
If the suspicion is confirmed, the provisions could be tightened again.
A passenger aboard the ship sent the following information:
"I'm on the Royal Caribbean's jewel of the seas cruise ship. The Dubai government has instructed the ship to return back to port. One passenger has been sent to hospital for medical testing. All other passengers cannot disembark. Cruise itinerary has been cancelled."
UPDATE: Hours later, the same passenger wrote:
"Patient result came negative and Dubai authorities gave permission for passengers to disembark and use the ship as a hotel. Cruise itinerary cancelled. Many pasangers leaving the ship to return to their country of origin. All Arabian gulf state ports not excepting cruise ships."
For other Jewel of the Seas accidents and incidents see at the ship's CruiseMinus page.
For Coronavirus updates on cruise ship quarantines (infected passengers and crew) and top-pandemic countries (COVID-19 cases and deaths, daily updated statistics) see at CruiseMapper's Norovirus page.