The cruise company said each person who had tested positive on arrival immediately went into quarantine. It said six people who had tested positive onboard had already disembarked from the ship mid-voyage.
It added that those who had tested positive were either asymptomatic or had mild symptoms of the virus.
The Symphony of the Seas ship is the largest cruise liner in the world and has featured in documentaries.
The boat departed on December 11 with 6,091 passengers onboard. It said 95 percent of passengers were fully vaccinated.
In total, The Symphony of the Seas has capacity for 9,000 guests and crew onboard.
Royal Caribbean said it had enforced strict rules for all passengers travelling on the voyage.
Every passenger over the age of 12 had to be fully vaccinated to board and test negative before departure.
One cruise guest James O’Dell told Miami Herald that he wasn’t convinced by the company’s safety procedures.
He told the paper that two members of his holiday party had tested positive for Covid on the boat.
He said: “I bought into the safety aspect. I was reading the literature they have online and thought ‘how much safer can you get?’
“Everyone’s vaccinated and has to be tested. And then you get on board and find yourself in the middle of an outbreak.”
According to the Miami Herald, Symphony of the Seas planned schedule will not impacted by the outbreak.
The boat has already departed Miami on a cruise to Cozumel in Mexico and is expected to arrive on December 20.
A spokesperson for Royal Caribbean said: “The 48 passengers were identified as a result of immediate contact tracing after a guest tested positive.
“The cruise sailed with 95 percent of the onboard community fully vaccinated and 98 percent of those who tested positive were fully vaccinated.
“Each person quickly went into quarantine. Everyone who tested positive was asymptomatic or mildly symptomatic and we continuously monitored their health.”
Royal Caribbean strongly recommends passengers have a booster jab before the cruise but it is not mandatory.
Cruises have had a very fraught time during the pandemic with some calling them “floating petri dishes”.
However, the boats have been allowed back on the seas since July with many travellers taking to the water for a holiday.
Most cruise lines have implemented new Covid rules such as facemasks and increased use of sanitiser.
In August, 27 people, including 26 crew tested positive for Covid on a Carnival cruise in Belize.