Members of media gather at the Diamond Princess cruise ship, operated by Carnival Corp., docked in Yokohama, Japan, on Friday, Feb. 7, 2020.

Cruise lines are among the businesses that have suffered the most fallout from the coronavirus outbreak. And the major ones could get left out of the bailout fund included in the $2 trillion stimulus bill the Senate passed Wednesday night.

The bill allocates $500 billion to distressed businesses that can apply for loans or guarantees from a fund overseen by Treasury Secretary Steven Mnuchin. To be eligible for that relief, however, a company must be "created or organized in the United States or under the laws of the United States" and "have significant operations in and a majority of its employees based in the United States."

Several major cruise lines, however, are not incorporated in America. Carnival, for example, is incorporated in Panama, although it has a U.S. entity and a headquarters in Miami. Its shares trade on the New York Stock Exchange. Cruise lines also typically employ many foreign workers on their ships, who can be exempt from U.S. minimum wage requirements.

"My interpretation is that cruise lines may not qualify, but I'm taking a closer look to see whether that is really the case," said Aaron Cutler, a partner in the government relations and public affairs department at influential international law firm Hogan Lovells.

In a statement early Thursday evening, industry trade group Cruise Lines International Association said they are "grateful that more than 30,000 CLIA travel agent members, most of which operate small and medium-sized businesses, can receive much-needed relief under the bill."

"For the more than 421,000 people in the United States whose jobs are supported by the cruise industry, we will continue to work with policymakers to help our community recover from the impact of this pandemic. We congratulate the Administration and Senate Leaders for reaching a historic agreement to address the unprecedented crisis that has gripped our nation and the world," they added.

Royal Caribbean, which is incorporated in Liberia, referred requests for comment to CLIA. Carnival declined to comment. Norwegian Cruises, incorporated in Bermuda, did not respond to a request for comment. Several congressional aides did not return requests for comment, as well.

Unlike restrictions around dividends and buybacks in the stimulus bill, the Treasury Department has no discretion to waive restrictions around whether offshore incorporation exempts a company from relief.

The exclusion of cruise lines from relief would come as a surprise. President Donald Trump has indicated he would help the ailing industry, along with hotels and airlines, which have all bared the brunt of restrictions on travel. Vice President Mike Pence met with executives in the industry earlier this month.

Speaking at a coronavirus task force briefing Thursday, Trump said he liked the idea of cruise lines registering in the U.S. to get aid.

"It's a big business, it's a great business," he said.

"We're going to work very hard on the cruise line business and we're going to figure something out."

The idea of cruise lines registering in the U.S. had been floated earlier in the day by Sen. Josh Hawley, R-Mo.

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The industry was ground to halt by the coronavirus pandemic, after a number of outbreaks on ships. The Centers for Disease Control and Prevention earlier this week said it traced infections to more than 25 cruise ships. Several ships had to quarantine passengers. One Carnival-owned ship, the Grand Princess, was forced to moor off the coast of California when 21 people tested positive for the virus. Another Carnival-owned ship had at least 700 passengers infected, and at least eight died.

Carnival, Royal Caribbean and Norwegian Cruises have all suspended operations. The industry has high fixed costs, which means a pause in revenue could have a devastating impact on its balance sheets.

Shares of Carnival, the largest of the three cruise lines by sales, has a market capitalization of $12 billion after its shares have fallen nearly 65% year to date. Norwegian's shares are down 73%, giving it a market cap of $3.3 billion. Royal Caribbean's shares are down 69%, giving it an $8.5 billion market cap.

Still, unlike the airline industry, which was promised relief in the $2 trillion stimulus bill, cruise lines are not considered a pillar of the U.S. economy. And unlike the airline industry, which has a huge national footprint with hubs throughout the country and smaller communities dependent upon flight service, there are only so many U.S. ports.

The industry has found critics in Congress over its safety standards and foreign incorporation.

Last year, Reps. Doris Matsui, D-Calif., and Jeff Fortenberry, R-La., introduced the Cruise Passenger Protection Act, which would require the industry to have a qualified physician and sufficient medical staff on board, as well as elevate reporting requirements for crimes that occur on ships.

Matsui earlier this month called on the chairs of the House Ways and Means Committee and Committee on Transportation and Infrastructure to block a bailout of the industry.

"Unlike American families, corporations that operate foreign-flagged vessels do not pay taxes in the United States," she wrote. "Additionally, pervasive safety and medical shortcomings on vessels have caused numerous Americans harm. "

"Directing federal funding to the cruise industry during a public health crisis will not slow the spread of the coronavirus nor advance passenger safety," she added.

-- CNBC's William Feuer contributed to this report.