People from around the world put money in the stock exchange. That isn't just an American thing. And competant investors diversify their portfolios to ensure that a downturn in one industry won't devastate their stock portfolios.
Carnival and RCL going under will be devastating to the corporate boards of Carnival and RCL, to their mostly foreign crews, a few thousand U.S. port workers, and some small businesses in port towns in Alaska. That's it, and that's not enough to justify a massive taxpayer-funded bailout.
It will not cause a "domino effect" on tourism throughout Florida, which is a mecca for tourism for many other reasons (theme parks, beaches, the Keys, etc.) It will barely be a blip on the radar of Texas and California, which are massive states with highly diversified economies and tourist opportunities that are mostly unrelated to cruising.
Shareholder loss is not the issue here. Every equity investment carries this risk, and Carnival and RCL are no exceptions.
Now, for 'few thousand port workers', there are more than 140,000 people employed in Florida's port sector at the moment. This number doesn't include all those in related jobs such as accounting firms providing services to the ports. An average cruise ship employs about 1,000 crew, and there are a max of about 60 of them in Florida in the peak season (down to less than 20 in the summer).
See page 8 for a summary of these numbers:
http://scdn.flaports.org/wp-content/uploads/EconomicImpactsofFloridaSeaports.pdf
The big ports in Florida (Miami, PC, Everglades) have more than 15 million cruise passengers passing through them annually. That's almost 15% of the all tourists in Florida. On top of the port fees, the cruise guests spend on hotels, meals, transportation and shopping before/after the cruise. Those businesses then go on to employ staff to meet the demand, and the staff then spends on yet other goods and services.
Source of passenger numbers:
https://en.wikipedia.org/wiki/List_of_busiest_cruise_ports_by_passengers
Source of cruise arrival numbers:
http://crew-center.com/analysis-florida-cruise-statistics-ports-2017
None of this warrants a bailout. Cruise industry isn't an essential service - or a part of our physical/digital infrastructure. If the industry goes down because the demand disappears, that's just economics 101. We aren't bailing out malls either, as they die a slow death at the hands of
Amazon, Costco, and Walmart.
BUT - cruising is a fast-growing US vacation and travel choice, and it does have a substantial impact on certain parts of the economy. There are creative ways to help this industry - any industry - if it has taken a disproportionate hit due to something we all agree we need to fight.