Unlike airlines and hotels, cruise ships are having a hard time getting business restarted this year.

Now even one so-called "cruise to nowhere" has found itself curtailed. Royal Caribbean's Quantum of the Seas passenger cruise ship cut short its four-day itinerary on Wednesday, returning to Singapore a day early after an 83-year-old male passenger tested positive for Covid-19.

The cruise is part of a pilot scheme launched in November that allowed two cruise lines — Royal Caribbean and Genting Cruise Lines' Dream Cruises — to operate short, round-trip cruises with no ports of call and mandatory Covid-19 testing for all on board.

A 'hole' in the system

Passengers were required to take a PCR (polymerase chain reaction) test 48 to 72 hours before embarking, said Heidi Sarna, a cruise expert and co-founder of travel site Quirky Cruise who is currently onboard the diverted cruise ship.

"I went on Friday to get my test and got [the results] on Saturday by email," she told CNBC. "The cruise left on Monday."

Sarna acknowledged this created "a hole" in the safety protocol.

"I take the test, then I'm running around for a few days before the cruise," she said.

Sarna highlights a problem many in the travel industry face as they try to use testing to weed out Covid infections on flights and in hotels. Royal Caribbean is relying on PCR tests, which must be administered early for results to be processed in a lab. Dream Cruises is using antigen tests on the date of departure. Though less accurate, those antigen tests produce passenger results in about an hour, according the company's website.

Still, Sarna was surprised when she learned of the infected passenger. Singapore, a country of 5.7 million people, has averaged fewer than one case per day over the past two weeks, a number which does not include imported cases that are quarantined upon arrival.

"It's odd because there are virtually no cases in Singapore," she said. "That's the mystery."

Announcement in the middle of the night

Around 1:50 a.m., ship captain Sindre Borsheim announced the ship was going into quarantine mode and returning to Singapore, said Sarna.

"You're suddenly awakened by this voice," she said. "He didn't mention Covid in that first announcement, but everybody could figure out what that meant."

The ship docked around 8 a.m. in Singapore.   

Though passengers aboard the cruise ship don't know when they will depart, Sarna said she does not believe the ship will encounter the same fate as high-profile incidents where passengers and crew were stuck on cruise ships for weeks.

"It's a different situation," she said. "We are at a different point in the way that Covid is dealt with, and Singapore is Singapore."

She added that contract tracing on the ship has been "amazing." Passengers were required to tap cabin keycards throughout the ship, wear Royal Caribbean's "Tracelet" tracing wristband and use Singapore's Trace Together app, which works via Bluetooth.

"It was all very thorough," she said.

'The system is working'

Royal Caribbean confirmed passengers will be allowed to leave pending contact tracing results.  

"We identified and isolated all guests and crew who had close contact with [the positive Covid] guest, and each of those individuals have subsequently tested negative for the virus," said Jonathon Fishman, a company spokesman. "The ship has returned to port today in accordance with government protocols and will debark guests after a review of contact tracing is completed."

"That we were able to quickly identify this single case and take immediate action is a sign that the system is working as it was designed to do," he said.

Quantum of the Seas is carrying 1,688 passengers, said Fishman, less than half of the 4,000 passengers it has the capacity to hold.

For the most part, Sarna is upbeat. Passengers who have not been isolated must pass an antigen test before disembarking, which Sarna said was always a requirement.

"Then we are all free to go home," she said.

Around 3 p.m. today local time, Captain Borsheim updated the passengers that the ill passenger had been removed from the ship and that the cruise line is working with the Singapore government and Singapore Tourism Board to facilitate a disembarkation for all remaining passengers. That process may not necessarily be imminent, as he also announced dinner would be served at 7 p.m.

The problems with testing

The passenger who tested positive had tested negative prior to boarding, said Annie Chang, director for cruise at the Singapore Tourism Board.

Though Covid-19 tests are increasingly being relied on to revive the embattled travel industry, testing isn't failproof. Covid-19 infections are not easily detected during early stages of the disease, and tests can fail to detect active cases too.

Rapid PCR tests, which are both accurate and fast, exist but production is outmatched by demand, said Dr. Shira Doron, infectious disease physician and hospital epidemiologist at Tufts Medical Center.

And even when tests work, passengers may slip through the cracks. Last week, a couple traveled to Hawaii after they both tested positive as part of United Airline's preflight testing program. The pair has since been charged with reckless endangerment and banned from flying with United.