Bundle-up season has arrived in much of the country, which means many travelers are thinking about where they can go to get warm.

And this year — unlike last, when the industry was still shut down in the United States — cruise ships are an option. According to the Centers for Disease Control and Prevention, 81 vessels are operating or planning to operate in U.S. waters.

As cruise companies return more of their fleets to service, they are operating under a new normal. That typically means fewer passengers, vaccine requirements, mandatory testing and masks in crowded indoor spaces. That’s actually good news for many cruise fans.

“I feel far safer on a cruise ship than I do visiting my local grocery store or flying or even staying at a hotel,” said Colleen McDaniel, editor in chief of the cruise news and review site Cruise Critic. “Because on a cruise ship, I know that the people I am sailing with have had to undergo testing and they’re vaccinated.”

On Sunday, McDaniel is heading out on her 10th cruise since the pandemic started, and she said she doesn’t think she’s been on a ship that was more than 60 percent full because of capacity limits. While some rules are consistent under public health guidelines, other requirements and experiences on board vary according to cruise line and destination. Here’s what travelers should know.

Are cruise ships sailing again from the U.S.?

Many are — just not as many as before the pandemic. The industry started cranking back to life in the United States with just one ship in late June, but that number has ballooned in the past few months. Now, dozens of ships are heading out from ports in states including Florida, California, Texas, Louisiana, Maryland and New York. Ships are mostly visiting the Caribbean, Bahamas, Mexico and Bermuda.

More are joining soon. Carnival Cruise Line, for example, has 13 ships sailing in the United States now, but it plans to add another four by the end of 2021. Princess Cruises is operating three ships in the United States, a number that will jump to eight by year’s end. Royal Caribbean International says its entire 26-ship fleet will be back in service by spring of 2022.

Globally, the Cruise Lines International Association said 55 percent of oceangoing capacity among its member lines had been activated by the end of September. The group expects nearly 80 percent of oceangoing capacity to be in operation by the end of the year.

Are ships at full capacity again?

No, but many are working up to it. Most operators started with significantly reduced capacity to give passengers more space. Now, the amount of crowding depends on the line — though not all companies are providing specific information about their passenger limits.

Carnival spokesman Chris Chiames said in an email that the line is still “at a somewhat limited capacity” but that will expand in the coming months. Norwegian Cruise Line’s ships are sailing at 50 to 60 percent capacity. Newcomer Virgin Voyages has its Miami-based ship capped at 50 percent. And Princess Cruises is operating between 60 and 75 percent full, with plans to increase that every voyage.

Do I have to be vaccinated to take a cruise?

If you’re 12 or older, the answer is almost certainly yes. Due to CDC guidelines, rules in certain destinations and cruise lines’ own policies, vaccines have been widely required. Some cruise lines allow limited exemptions, but travelers should check the policies in advance. Norwegian Cruise Line is not allowing any unvaccinated passengers on board, so children who are too young for the shot are not allowed to sail yet. Virgin Voyages’ Miami ship was set to be adult-only even before the pandemic.

It’s not clear how long the vaccine mandate for cruises will last. Most cruise lines have said they will be in place for the near future — through at least the end of December for Norwegian and end of February for Carnival and Princess. MSC Cruises says the rule depends on “the evolution of the pandemic” and location requirements, while Virgin says “there are no short-term plans to end this requirement.” To sum up: Travelers shouldn’t expect this rule to go away anytime soon.

What covid-19 safety measures are in place?

In addition to vaccination, passengers on most lines must test negative before boarding. The time frame for testing — and number of pre-cruise tests — is different depending on whether a traveler is vaccinated or not. Some companies offer testing at the port before departure, while others require passengers to provide proof of a negative test from an approved source.

Once on the ship, most cruise companies are requiring masks indoors unless passengers are in their own rooms, actively eating or drinking, or in vaccinated-only spaces. Norwegian does not require masks on U.S. ships because of its firm vaccine requirement for everyone on board. Virgin encourages face coverings indoors.

Can I still get off the ship in ports?

Yes, if you’re vaccinated. But what you can do in the port varies, especially if any members of your group aren’t vaccinated. Some destinations require organized tours. Others allow passengers to explore on their own. At least one — San Juan, Puerto Rico — requires unvaccinated people to stay on the ship. Chiames, of Carnival, said unvaccinated passengers who have received special permission to sail, including families with young children, have to take “bubble tours” in which the entire environment is controlled. On those Carnival-sponsored tours, guests are escorted to an activity and taken back to the ship without unscheduled stops at stores, restaurants or anywhere else.

Have there been coronavirus cases on ships?

Yes. Cruise lines did not provide a number of positive cases they have seen on board, but several cases have made news since ships started carrying customers again. One woman who tested positive during a Carnival cruise died, though the cruise line has said it doesn’t believe she caught it on the ship.

The CDC assigns colors to ships operating in the United States based on whether there have been reported cases of covid-19 on board. On Oct. 19, the chart showed the agency was monitoring or investigating 24 ships — of 81 sailing with passengers or just crew — for reported cases. No ships have reached the level of cases where it would need to return to port or delay a trip, the cruise association said, based on CDC data.

“The relatively rare instances of covid-19 that have occurred since operations resumed have been addressed swiftly based upon prearranged response plans onboard every CLIA oceangoing member cruise ship,” association spokeswoman Bari Golin-Blaugrund said in an email.

Before cruising shut down in March 2020 — and after, as ships with ill passengers desperately tried to return to land — the industry dealt with major outbreaks on several ships.

“People on a large ship, all together, at the same time, all the time — you couldn’t ask for a better incubator for infection,” Anthony S. Fauci, the country’s top infectious-diseases expert, said in February 2020.

What does the CDC say about cruising now?

The public health agency lists cruising at a “Level 3” risk which means “high level of covid-19.” On its website, the CDC says unvaccinated people should avoid cruising, as should anyone at risk of severe illness from covid-19, whether they are vaccinated or not.

“The virus that causes covid-19 spreads easily between people in close quarters aboard ships, and the chance of getting covid-19 on cruise ships is high,” the agency says on the site.

Officials recommend people get tested one to three days before a trip and three to five days after, whether they are vaccinated or not. During a cruise, travelers should wear a face covering in shared spaces, according to public health recommendations. Those who aren’t vaccinated should self-quarantine for seven days after cruising, even if they test negative, the CDC says.

What if I have a cruise booked but I’m not ready to go yet?

Check the fine print on your purchase carefully. While many cruise lines have flexible cancellation or rescheduling policies, some are more forgiving than others. Some policies only apply to bookings made through a certain date or departing by a certain time. And the time frame when travelers need to make their change varies by line.

For the most part, travelers will get a credit for a future cruise if they are able to change their plans.