Section 381.00316 is unconstitutional because it is an unjustifiable content-based restriction on speech, in violation of the First Amendment.
The law is unconstitutional because private businesses and their owners have a First Amendment right to demand that their customers engage in speech as a condition of doing business.
If you've watched Showtime at the Apollo or America's Got Talent, you understand the concept that a business owner can make decisions -- objectively or subjectively -- about what kinds of speech they want to host. If you want to make it through your performance, it's incumbent on you to satisfy the predilections of the business owner.
And this doesn't just apply to TV. If you went to a poetry slam at your local coffee shop, the owners could boot you if talked through the performances, making it difficult for other customers to enjoy the show. Likewise, they could boot you if you went to the mic and started spouting racist nonsense, or if you just stood at the mic and said nothing at all.
In the Florida case, Norwegian is doing the same thing -- just not for artistic reasons. Beyond requiring customers to pay for a ticket, it is requiring its customers to communicate the message that they are vaccinated, and they are requiring them to communicate using written documentation. In other words, Norwegian is demanding that customers engage in speech ("I am vaccinated") in a certain way (written proof of vaccination), and that kind of exchange falls within the protections of the First Amendment.
That speech is subject to regulation because of its vaccination-related content of that speech, making Section 381.00316 a content-based restriction on speech, which subjects it to strict scrutiny, meaning that the state cannot enforce the law unless it furthers a compelling governmental interest and is narrowly tailored to advance that interest.
The state argued that the law furthered compelling governmental interests in protecting medical privacy and preventing discrimination, but the court wasn't buying it:
Here, Defendant has presented no evidence to demonstrate that his asserted interests are in response to real problems that Florida residents are actually facing. There is no evidentiary support to show that residents have experienced intrusions on their medical privacy or discrimination because some businesses, including cruise lines, have required COVID-19 vaccination documentation. The legislative record cited by Defendant is bereft of any facts or data underpinning the Statute’s purported purpose. In light of the absence of any appropriate data, reports, or even anecdotal evidence on this issue, the Court cannot conclude that Defendant’s articulated interests are based on a problem that exists in fact.
And even if these were compelling interests, the state failed to prove that the law was actually advancing them. Because the law outlawed the exchange of written communication rather than outlawing the vaccination requirements themselves, businesses remained free to demand information about vaccination status (through oral statements, for instance), and to discriminate against who they knew or believed to be unvaccinated.
(N.B.: For procedural reasons, they court actually applied intermediate scrutiny, but the law ends up invalid for basically the same reasons.)
Section 381.00316 is unconstitutional because it substantially burdens interstate commerce in violation of the Dormant Commerce Clause.
The law was also struck down because it intrudes on the federal government's authority to regulate interstate (and international) commerce.
When a law is challenged under the Dormant Commerce Clause, courts ask two questions: (1) Does the state law favor in-state economic interests; and (2) Does the state law's burden on interstate commerce outweigh the legitimate local benefits to the state?
A "yes" answer to either will invalidate the law. Here, the court held that the law did not directly favor in-state economic interests, but that the burdens on commerce outweighed the local benefits. Because the law did not meaningfully advance any of the local interests it had invoked (as discussed above), but it did impose a substantial burden on interstate commerce because Norwegian's ships travel to additional ports where proof of vaccination is required:
Section 381.00316 will prevent NCLH and other cruise lines from possessing verified information necessary to effectively and efficiently process landing and disembarking at various, preferred domestic and international ports where documentary proof of vaccination is required. This affects not only opportunities for vacation activities like sightseeing, but also responses to mechanical and medical emergencies, or even geopolitical crises. Depriving cruise lines of corroboration of passengers’ vaccination status impedes their ability to prepare and address these eventualities.
Either the First Amendment violation or the Commerce Clause violation would have been independently enough to invalidate the statute. Norwegian also argued that the statute was preempted by the CDC's Conditional Sailing Order and related orders, but the court declined to address that argument.