Some major cruise lines will now require pre-boarding testing and masks to be worn in certain indoor areas -- even for vaccinated guests.

"We have seen a number of ships report some isolated cases of COVID," said Cruise Critic Editor-in-Chief Colleen McDaniel. "And what we've seen is these have been mostly among vaccinated passengers, and certainly the delta variant seems to be having an effect on that."

Despite at least 95% of guests and crew being vaccinated, Carnival Vista, which departed out of Galveston, Texas, reported a "small number of positive cases" this week -- prompting the cruise line to change their policy.

The positive cases are in isolation, but the new mask-wearing rule will begin immediately on the Vista.

Carnival's policy will go into effect after Aug. 7 through Oct. 31 for other sailings.

"These new requirements are being implemented to protect our guests and crew while on board, and to continue to provide confidence to our homeports and destinations that we are doing our part to support their efforts to protect public health and safety," Carnival Cruise Line President Christine Duffy said in a statement. "We expect these requirements will be temporary and appreciate the cooperation of our guests."

Holland America and Princess Cruises, which are both owned by Carnival Corporation, announced the same new cruising requirements.

In addition to masking, all fully vaccinated guests will be required to show proof of a negative COVID-19 test taken within three days of their embarkation.

"I think the requirements are going to continue to shift based on how we're seeing the delta variant affect our day-to-day lives, both at home and also as we travel," McDaniel said. "I think this is going to stick while we figure out how the delta variant is going to affect us, day to day."

But some experts don't believe the new rules will deter future passengers.

"What we have found is that cruise passengers want to cruise and that they want to cruise safely," Cruise Critic Managing Editor Chris Gray Faust told ABC News. "So no matter what they're doing, they're going to be following the cruise line protocols in order so that they can stay safe and feel like they're having a great vacation."

Dr. Anne Rimoin, an epidemiology professor at the University of California, Los Angeles, recommended people determine what their risk threshold is before deciding to take a cruise in light of the growing number of cases.

"You have to think very carefully," she said. "Do you have someone in your household, who isn't vaccinated, that is at risk? Do I have small children that could become infected if I get infected?"

She said to keep in mind how contagious the delta variant is, and that it is easier to get COVID now than it has ever been.

"Vaccinated or not, you have to think about your risk, you have to think about where you are, who you're around and mask accordingly," she explained.

ABC News' Sam Sweeney and Amanda Maile contributed to this report.