(CNN) -- Hundreds of people aboard two cruise ships in the Caribbean fell ill due to norovirus, the latest instances in which the stomach bug has thwarted vacationers at sea.
The Centers for Disease Control and Prevention reported Friday that norovirus had hit more than 600 passengers and crew on Royal Caribbean's Explorer of the Seas, making it one of the biggest such outbreaks on any cruise ship in the last two decades.
The vessel returned to port Wednesday, two days earlier than expected, and four days after the CDC began its investigation.
"No particular source has been identified, and it's quite possible a source won't be identified," the agency said.
In addition, the Caribbean Princess, operated by Princess Cruises, cut short its own seven-day Caribbean itinerary on Friday.
Princess Cruises said that 178 passengers and 11 crew members aboard had been stricken with norovirus, with CDC staffers on board Friday to help sanitize that ship.
The cruise line's official reason for ending the trip early was not the norovirus outbreak, but rather a forecast of dense fog this weekend at the Houston port that was its final call.
The report didn't sit well with some passengers, who spoke to CNN affiliate KHOU as they disembarked the ship Friday. Princess spokeswoman Julie Benson insisted that the prospect of fog making ship channels into the Houston port unnavigable was the only factor in turning the boat around.
Keith Davis of Austin questioned the fog story.
"That's last Tuesday they tell us this, and they've never said anything different," he told KHOU. "I knew then nobody can predict five days in advance. I knew that that wasn't the case. They were just doing that to minimize their damages."
Bill and Linda Askins of West Columbia, 50 miles south of Houston, said they had a great time but were disappointed to turn back before the ship reached Belize.
"I'm still looking for the fog," Bill Askins told the station. "It's a little irritating."
Linda Askins added, "We were looking forward to (going to Belize). That's why we had taken the cruise, yes, but things happen. We understand."
Port of Houston Authority spokesman Bill Hensel said Princess was informed Tuesday that there was "a high probability of a fog event" this weekend. ImpactWeather, which bills itself as a "full-time weather department for hundreds of companies around the globe," sent Princess a report Wednesday predicting dense fog setting in late Friday afternoon, Benson said.
"Breaks in the fog are not likely until the cold front moves offshore late Sunday morning or early Sunday afternoon. Thus, we are expecting that the ship channel will be closed for parts of Friday, all day Saturday, and Sunday through the afternoon hours," the report said.
Princess, concerned the boat "couldn't navigate the channel to get back to port," then made the decision to turn the boat around, Benson said. Asked if the ship would have continued on to Belize if the forecast had called for clear skies in Houston on Friday, she that was "100% correct."
The Caribbean Princess departed January 25 on its Western Caribbean tour with planned stops in Cozumel, Mexico, the Honduran island of Roatan and Belize, but threats of fog forced the ship to return home before it made it to Belize, Princess said in its statement. The cruise ship was carrying 3,104 passenger and 1,149 crew members.
"We truly regret having to make this change to our passengers' vacations, and we hope they understand that we did not have any choice but to return to Houston early before the unusually heavy fog closes the port," it said.
Princess will provide hotels for those flying out of Houston, and passengers "will receive a future cruise credit of 20% of their fare as well as one day per diem" for meals and other expenses, the statement said.
"Approximately five passengers" still had norovirus symptoms Friday, the cruise line said, describing it as "a common but contagious illness which is widely circulating throughout North America." The gastrointestinal illness causes vomiting and diarrhea, the CDC says.
Sick passengers were asked to stay in their cabins while staff disinfected public areas such as restrooms and elevators. Several passengers told KHOU that crew members wearing gloves distributed hand sanitizer throughout the ship, and passengers weren't allowed to touch items like salt shakers, tongs and forks in the ship's common areas.
Robert Fisher of Houston said he came down with an illness, but the symptoms passed after two days. Still, he said, the staff made him feel as if he were being incarcerated.
"I was tired of being held as a prisoner in the room," he said. "They didn't bring food. They said they were bringing food. We had to call and remind them every time."
His cabin mate, Steve Juneau said, "I would have to leave the room and go get him food." He added that the staff worked diligently to keep the outbreak from spreading.
"They were trying really hard, but they were also very rude about it," Juneau told KHOU.
Fisher said the 20% credit on a future cruise amounted to "nothing" when you consider he wasn't allowed to dine in the onboard restaurants and never saw Belize.
Despite more than one of every 20 passengers being ill, many passengers reported seeing no sick people during the trip. Lloyd and Rosie Swayne of Grand, Texas, saw a passenger vomiting on the beach during one of the cruise's stops.
"They immediately removed her, cleaned the area, bleached the sand, everything. It was handled well," Lloyd Swayne told KHOU.
CNN's Suzanne Presto, Dana Ford and Miriam Falco contributed to this report.